Cells: The Living Units

Transcription

1 Overview of the Cellular Basis of Life (pp. 62 6) The Plasma Membrane: Structure (pp. 6 67) The Fluid Mosaic Model (pp. 6 66) Membrane Junctions (pp ) The Plasma Membrane: Membrane Transport (pp ) Passive Processes (pp ) Active Processes (pp ) The Plasma Membrane: Generation of a Resting Membrane Potential (pp ) The Plasma Membrane: Cell- Environment Interactions (pp ) Roles of Cell Adhesion Molecules (pp ) Roles of Membrane Receptors (p. 81) Role of Voltage-Sensitive Membrane Channel Proteins (p. 81) The Cytoplasm (pp ) Cytoplasmic Organelles (pp. 8 89) Cellular Extensions (pp ) The Nucleus (pp ) The Nuclear Envelope (pp. 91 9) Nucleoli (p. 9) Chromatin (pp. 9 95) Cell Growth and Reproduction (pp ) The Cell Life Cycle (pp ) Protein Synthesis (pp ) Other Roles of DNA (pp ) Cells: The Living Units Just as bricks and timbers are the structural units of a house, cells are the structural units of all living things, from one-celled generalists like amoebas to complex multicellular organisms such as humans, dogs, and trees. The human body has 50 to 100 trillion of these tiny building blocks. This chapter focuses on structures and functions shared by all cells. We address specialized cells and their unique functions in later chapters. Cytosolic Protein Degradation (pp ) Extracellular Materials (p. 107) Developmental Aspects of Cells (pp ) 61

2 62 UNIT 1 Organization of the Body Overview of the Cellular Basis of Life Define cell. List the three major regions of a generalized cell and indicate the function of each. Fibroblasts Erythrocytes The English scientist Robert Hooke first observed plant cells with a crude microscope in the late 1600s. However, it was not until the 180s that two German scientists, Matthias Schleiden and Theodor Schwann, were bold enough to insist that all living things are composed of cells. The German pathologist Rudolf Virchow extended this idea by contending that cells arise only from other cells. Virchow s proclamation was revolutionary because it openly challenged the widely accepted theory of spontaneous generation, which held that organisms arise spontaneously from garbage or other nonliving matter. Since the late 1800s, cell research has been exceptionally fruitful and provided us with four concepts collectively known as the cell theory: 1. A cell is the basic structural and functional unit of living organisms. So when you define cell properties you are in fact defining the properties of life. 2. The activity of an organism depends on both the individual and the collective activities of its cells.. According to the principle of complementarity of structure and function, the biochemical activities of cells are dictated by the relative number of their specific subcellular structures. 4. Continuity of life from one generation to another has a cellular basis. We will expand on all of these concepts as we progress. Let us begin with the idea that the cell is the smallest living unit. Whatever its form, however it behaves, the cell is the microscopic package that contains all the parts necessary to survive in an ever-changing world. It follows then that loss of cellular homeostasis underlies virtually every disease. The trillions of cells in the human body include over 200 different cell types that vary greatly in shape, size, and function (Figure.1). The spherical fat cells, disc-shaped red blood cells, branching nerve cells, and cubelike cells of kidney tubules are just a few examples of the shapes cells take. Depending on type, cells also vary greatly in length ranging from 2 micrometers (1/12,000 of an inch) in the smallest cells to over a meter in the nerve cells that cause you to wiggle your toes. A cell s shape reflects its function. For example, the flat, tilelike epithelial cells that line the inside of your cheek fit closely together, forming a living barrier that protects underlying tissues from bacterial invasion. Regardless of type, all cells are composed chiefly of carbon, hydrogen, nitrogen, oxygen, and trace amounts of several other elements. In addition, all cells have the same basic parts and some common functions. For this reason, it is possible to speak of a generalized,or composite, cell (Figure.2). Human cells have three main parts: the plasma membrane, the cytoplasm, and the nucleus. The plasma membrane, a fragile Epithelial cells (a) Cells that connect body parts, form linings, or transport gases Skeletal muscle cell (b) Cells that move organs and body parts Fat cell (c) Cell that stores nutrients Nerve cell Macrophage (d) Cell that fights disease (e) Cell that gathers information and controls body functions (f) Cell of reproduction Sperm Smooth muscle cells Figure.1 Cell diversity. (Note that cells are not drawn to the same scale.) barrier, is the outer boundary of the cell. Internal to this membrane is the cytoplasm (si to-plazm), the intracellular fluid that is packed with organelles, small structures that perform specific cell functions. The nucleus (nu kle-us) controls cellular activities and typically it lies near the cell s center. We use these three main parts of the cell to organize the summary in Table. (pp ), and we describe them in greater detail next.

3 Chapter Cells: The Living Units 6 Chromatin Nuclear envelope Nucleolus Nucleus Smooth endoplasmic reticulum Plasma membrane Cytosol Mitochondrion Lysosome Centrioles Centrosome matrix Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Figure.2 Structure of the generalized cell. No cell is exactly like this one, but this composite illustrates features common to many human cells. Note that not all of the organelles are drawn to the same scale in this illustration. CHECK YOUR UNDERSTANDING 1. Name the three basic parts of a cell and describe the function of each. 2. How would you explain the meaning of a generalized cell to a classmate? For answers, see Appendix G. The Plasma Membrane: Structure Describe the chemical composition of the plasma membrane and relate it to membrane functions. Compare the structure and function of tight junctions, desmosomes, and gap junctions. The flexible plasma membrane defines the extent of a cell, thereby separating two of the body s major fluid compartments the intracellular fluid within cells and the extracellular fluid outside cells. The term cell membrane is commonly used as a synonym for plasma membrane, but because nearly all cellular organelles are enclosed in a membrane, in this book we will always refer to the cell s surface, or outer limiting membrane, as the plasma membrane. The plasma membrane is much more than a passive envelope. As you will see, its unique structure allows it to play a dynamic role in many cellular activities. The Fluid Mosaic Model The fluid mosaic model of membrane structure depicts the plasma membrane as an exceedingly thin (7 10 nm) structure

4 64 UNIT 1 Organization of the Body composed of a double layer, or bilayer, of lipid molecules with protein molecules plugged into or dispersed in it (Figure.). The proteins, many of which float in the fluid lipid bilayer,form a constantly changing mosaic pattern. The model is named for this characteristic. Membrane Lipids The lipid bilayer forms the basic fabric of the membrane. It is constructed largely of phospholipids, with smaller amounts of cholesterol and glycolipids. Each lollipop-shaped phospholipid molecule has a polar head that is charged and is hydrophilic (hydro water, philic loving), and an uncharged, nonpolar tail that is made of two fatty acid chains and is hydrophobic (phobia fear). The polar heads are attracted to water the main constituent of both the intracellular and extracellular fluids and so they lie on both the inner and outer surfaces of the membrane. The nonpolar tails, being hydrophobic, avoid water and line up in the center of the membrane. The result is that all biological membranes share a common sandwich-like structure: They are composed of two parallel sheets of phospholipid molecules lying tail to tail, with their polar heads exposed to water both inside and outside the cell. This self-orienting property of phospholipids encourages biological membranes to self-assemble into closed, generally spherical, structures and to reseal themselves quickly when torn. The plasma membrane is a dynamic fluid structure that is in constant flux. Its consistency is like that of olive oil. The lipid molecules of the bilayer move freely from side to side, parallel to the membrane surface, but their polar-nonpolar interactions prevent them from flip-flopping or moving from one phospholipid layer (half of the bilayer) to the other. The inward-facing and outward-facing surfaces of the plasma membrane differ in the kinds and amounts of lipids they contain, and these variations are important in determining local membrane structure and function. The majority of membrane phospholipids are unsaturated, a condition which kinks their tails (increasing the space between them) and increases membrane fluidity. (See the illustration of phosphatidylcholine in Figure 2.16b, p. 46.) Glycolipids (gli ko-lip idz) are lipids with attached sugar groups. They are found only on the outer plasma membrane surface and account for about 5% of the total membrane lipid. Their sugar groups, like the phosphate-containing groups of phospholipids, make that end of the glycolipid molecule polar, whereas the fatty acid tails are nonpolar. Some 20% of membrane lipid is cholesterol. Like phospholipids, cholesterol has a polar region (its hydroxyl group) and a nonpolar region (its fused ring system). It wedges its platelike hydrocarbon rings between the phospholipid tails, stabilizing the membrane, while increasing the mobility of the phospholipids and the fluidity of the membrane. About 20% of the outer membrane surface contains lipid rafts, dynamic assemblies of saturated phospholipids (which pack together tightly) associated with unique lipids called sphingolipids and lots of cholesterol. These quiltlike patches are more stable and orderly and less fluid than the rest of the membrane, and they can include or exclude specific proteins to various extents. Because of these qualities, lipid rafts are assumed to be concentrating platforms for certain receptor molecules or for molecules needed for cell signaling. (Cell signaling will be discussed on pp ) Membrane Proteins Proteins make up about half of the plasma membrane by mass and are responsible for most of the specialized membrane functions. There are two distinct populations of membrane proteins, integral and peripheral (Figure.). Integral proteins are firmly inserted into the lipid bilayer. Some protrude from one membrane face only, but most are transmembrane proteins that span the entire width of the membrane and protrude on both sides. Whether transmembrane or not, all integral proteins have both hydrophobic and hydrophilic regions. This structural feature allows them to interact both with the nonpolar lipid tails buried in the membrane and with water inside and outside the cell. Although some are enzymes, most transmembrane proteins are involved in transport. Some cluster together to form channels, or pores, through which small, water-soluble molecules or ions can move, thus bypassing the lipid part of the membrane. Others act as carriers that bind to a substance and then move it through the membrane. Still others are receptors for hormones or other chemical messengers and relay messages to the cell interior (a process called signal transduction) (Figure.4a, b). Peripheral proteins (Figure.), in contrast, are not embedded in the lipid. Instead, they attach rather loosely only to integral proteins and are easily removed without disrupting the membrane. Peripheral proteins include a network of filaments that helps support the membrane from its cytoplasmic side (Figure.4c). Some peripheral proteins are enzymes. Others are motor proteins involved in mechanical functions, such as changing cell shape during cell division and muscle cell contraction. Still others link cells together. Some of the proteins float freely. Others, particularly the peripheral proteins, are restricted in their movements because they are tethered to intracellular structures that make up the cytoskeleton. Many of the proteins that abut the extracellular fluid are glycoproteins with branching sugar groups. The term glycocalyx (gli ko-kal iks; sugar covering ) is used to describe the fuzzy, sticky, carbohydrate-rich area at the cell surface. You can think of your cells as sugar-coated. The glycocalyx that clings to each cell s surface is enriched both by glycolipids and by glycoproteins secreted by the cell. Because every cell type has a different pattern of sugars in its glycocalyx, the glycocalyx provides highly specific biological markers by which approaching cells recognize each other (Figure.4f). For example, a sperm recognizes an ovum (egg cell) by the ovum s unique glycocalyx. Cells of the immune system identify a bacterium by binding to certain membrane glycoproteins in the bacterial glycocalyx.

5 Chapter Cells: The Living Units 65 Polar head of phospholipid molecule Cholesterol Extracellular fluid (watery environment) Glycolipid Glycoprotein Nonpolar tail of phospholipid molecule Carbohydrate of glycocalyx Bimolecular lipid layer containing proteins Outward-facing layer of phospholipids Inward-facing layer of phospholipids Cytoplasm (watery environment) Integral proteins Filament of cytoskeleton Peripheral proteins Figure. Structure of the plasma membrane according to the fluid mosaic model. The lipid bilayer forms the basic structure of the membrane. The associated proteins are involved in membrane functions such as membrane transport, catalysis, and cell-to-cell recognition.

6 66 UNIT 1 Organization of the Body (a) Transport A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. HOMEOSTATIC IMBALANCE Definite changes in the glycocalyx occur in a cell that is becoming cancerous. In fact, a cancer cell s glycocalyx may change almost continuously, allowing it to keep ahead of immune system recognition mechanisms and avoid destruction. (Cancer is discussed on pp ) ATP CHECK YOUR UNDERSTANDING Signal (b) Receptors for signal transduction A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell.. What basic structure do all cellular membranes share? 4. Why do phospholipids, which form the greater part of cell membranes, organize into a bilayer tail-to-tail in a watery environment? 5. What is the importance of the glycocalyx in cell interactions? For answers, see Appendix G. Receptor (c) Attachment to the cytoskeleton and extracellular matrix (ECM) Elements of the cytoskeleton (cell s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. (d) Enzymatic activity Membrane Junctions Although certain cell types blood cells, sperm cells, and some phagocytic cells (which ingest and destroy bacteria and other substances) are footloose in the body, many other types, particularly epithelial cells, are knit into tight communities. Typically, three factors act to bind cells together: 1. Glycoproteins in the glycocalyx act as an adhesive. 2. Wavy contours of the membranes of adjacent cells fit together in a tongue-and-groove fashion.. Special membrane junctions are formed (Figure.5). Enzymes A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here. (e) Intercellular joining Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions. Because junctions are the most important factor securing cells together, let us look more closely at the various types. Tight Junctions In a tight junction, a series of integral protein molecules (including occludins and claudins) in the plasma membranes of adjacent cells fuse together, forming an impermeable junction that encircles the cell (Figure.5a). Tight junctions help prevent molecules from passing through the extracellular space between adjacent cells. For example, tight junctions between epithelial cells lining the digestive tract keep digestive enzymes and microorganisms in the intestine from seeping into the bloodstream. (Although called impermeable junctions, some tight junctions are somewhat leaky and may allow certain types of ions to pass.) CAMs Glycoprotein (f) Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Figure.4 Membrane proteins perform many tasks. A single protein may perform some combination of these tasks. Desmosomes Desmosomes (des mo-sōmz; binding bodies ) are anchoring junctions mechanical couplings scattered like rivets along the sides of abutting cells that prevent their separation (Figure.5b). On the cytoplasmic face of each plasma membrane is a buttonlike thickening called a plaque.adjacent cells are held together by thin linker protein filaments (cadherins) that extend from the plaques and fit together like the teeth of a zipper in the intercellular space. Thicker keratin filaments (intermediate filaments, which form part of the cytoskeleton) extend from the cytoplasmic side of the plaque across the width of the cell to anchor to the plaque on the cell s opposite side. In this way, desmosomes not only bind neighboring cells together,

7 Chapter Cells: The Living Units 67 Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Plaque Intercellular space Interlocking junctional proteins Intercellular space (a) Tight junctions: Impermeable junctions prevent molecules from passing through the intercellular space. Intermediate filament (keratin) Linker glycoproteins (cadherins) (b) Desmosomes: Anchoring junctions bind adjacent cells together and help form an internal tension-reducing network of fibers. Channel between cells (connexon) (c) Gap junctions: Communicating junctions allow ions and small molecules to pass from one cell to the next for intercellular communication. Figure.5 Cell junctions. An epithelial cell is shown joined to adjacent cells by three common types of cell junctions. (Note: Except for epithelia, it is unlikely that a single cell will have all three junction types.) but they also contribute to a continuous internal network of strong guy-wires. This arrangement distributes tension throughout a cellular sheet and reduces the chance of tearing when it is subjected to pulling forces. Desmosomes are abundant in tissues subjected to great mechanical stress, such as skin and heart muscle. Gap Junctions A gap junction,or nexus (nek sus; bond ), is a communicating junction between adjacent cells. At gap junctions the adjacent plasma membranes are very close, and the cells are connected by hollow cylinders called connexons (kŏnek sonz), composed of transmembrane proteins. The many different types of connexon proteins vary the selectivity of the gap junction channels. Ions, simple sugars, and other small molecules pass through these water-filled channels from one cell to the next (Figure.5c). Gap junctions are present in electrically excitable tissues, such as the heart and smooth muscle, where ion passage from cell to cell helps synchronize their electrical activity and contraction. CHECK YOUR UNDERSTANDING 6. What two types of membrane junctions would you expect to find between muscle cells of the heart? For answer, see Appendix G.

8 68 UNIT 1 Organization of the Body Figure.6 Diffusion. Molecules in solution move continuously and collide constantly with other molecules, causing them to move away from areas of their highest concentration and become evenly distributed. From left to right, molecules from a dye pellet diffuse into the surrounding water down their concentration gradient. The Plasma Membrane: Membrane Transport Relate plasma membrane structure to active and passive transport processes. Compare and contrast simple diffusion, facilitated diffusion, and osmosis relative to substances transported, direction, and mechanism. Our cells are bathed in an extracellular fluid called interstitial fluid (in ter-stish al) that is derived from the blood. Interstitial fluid is like a rich, nutritious soup. It contains thousands of ingredients, including amino acids, sugars, fatty acids, vitamins, regulatory substances such as hormones and neurotransmitters, salts, and waste products. To remain healthy, each cell must extract from this mix the exact amounts of the substances it needs at specific times. Although there is continuous traffic across the plasma membrane, it is a selectively, or differentially, permeable barrier, meaning that it allows some substances to pass while excluding others. It allows nutrients to enter the cell, but keeps many undesirable substances out. At the same time, it keeps valuable cell proteins and other substances in the cell, but allows wastes to exit. Substances move through the plasma membrane in essentially two ways passively or actively. In passive processes,substances cross the membrane without any energy input from the cell. In active processes, the cell provides the metabolic energy (ATP) needed to move substances across the membrane. The various transport processes that occur in cells are summarized in Table.1 on p. 72 and Table.2 on p. 80. Let s examine each of these types of membrane transport. HOMEOSTATIC IMBALANCE Selective permeability is a characteristic of healthy, intact cells. When a cell (or its plasma membrane) is severely damaged, the membrane becomes permeable to virtually everything, and substances flow into and out of the cell freely. This phenomenon is evident when someone has been severely burned. Precious fluids, proteins, and ions weep from the dead and damaged cells. Passive Processes The two main types of passive transport are diffusion (difu zhun) and filtration. Diffusion is an important means of passive membrane transport for every cell of the body. Because filtration generally occurs only across capillary walls, that topic is more properly covered in conjunction with capillary transport processes later in the book. Diffusion Diffusion is the tendency of molecules or ions to move from an area where they are in higher concentration to an area where they are in lower concentration, that is, down or along their concentration gradient. The constant random and high-speed motion of molecules and ions (a result of their intrinsic kinetic energy) results in collisions. With each collision, the particles ricochet off one another and change direction. The overall effect of this erratic movement is the scattering or dispersion of the particles throughout the environment (Figure.6). The greater the difference in concentration of the diffusing molecules and ions between the two areas, the more collisions occur and the faster the net diffusion of the particles. Because the driving force for diffusion is the kinetic energy of the molecules themselves, the speed of diffusion is influenced by molecular size (the smaller, the faster) and by temperature (the warmer, the faster). In a closed container, diffusion eventually produces a uniform mixture of molecules. In other words, the system reaches equilibrium, with molecules moving equally in all directions (no net movement). Diffusion is occurring all around us, but obvious examples of pure diffusion are almost impossible to see. The reason is that any diffusion process that occurs over an easily observable distance takes a long time, and is often accompanied by other processes (convection, for example) that affect the movement of molecules and ions. In fact, one should suspect any readily observable example of diffusion. Nonetheless, diffusion is immensely important in physiological systems and it occurs rapidly because the distances molecules are moving are very short, perhaps 1/1000 (or less) the thickness of this page! Examples include the movement of ions across cell membranes and the movement of neurotransmitters between two nerve cells. The plasma membrane is a physical barrier to free diffusion because of its hydrophobic core. However, a molecule will diffuse

9 Chapter Cells: The Living Units 69 Extracellular fluid Lipidsoluble solutes Lipid-insoluble solutes (such as sugars or amino acids) Small lipidinsoluble solutes Water molecules Lipid billayer Cytoplasm Aquaporin (a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge (d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer Figure.7 Diffusion through the plasma membrane. through the membrane if the molecule is (1) lipid soluble, (2) small enough to pass through membrane channels, or () assisted by a carrier molecule. The unassisted diffusion of lipid-soluble or very small particles is called simple diffusion.a special name, osmosis, is given to the unassisted diffusion of a solvent (usually water) through a membrane. Assisted diffusion is known as facilitated diffusion. Simple Diffusion In simple diffusion, nonpolar and lipidsoluble substances diffuse directly through the lipid bilayer (Figure.7a). Such substances include oxygen, carbon dioxide, and fat-soluble vitamins. Because oxygen concentration is always higher in the blood than in tissue cells, oxygen continuously diffuses from the blood into the cells. Carbon dioxide, on the other hand, is in higher concentration within the cells, so it diffuses from tissue cells into the blood. Facilitated Diffusion Certain molecules, notably glucose and other sugars, some amino acids, and ions are transported passively even though they are unable to pass through the lipid bilayer. Instead they move through the membrane by a passive transport process called facilitated diffusion in which the transported substance either (1) binds to protein carriers in the membrane and is ferried across or (2) moves through waterfilled protein channels. Carriers are transmembrane integral proteins that show specificity for molecules of a certain polar substance or class of substances that are too large to pass through membrane channels, such as sugars and amino acids. The most popular model for the action of carriers indicates that changes in the shape of the carrier allow it to first envelop and then release the transported substance, shielding it en route from the nonpolar regions of the membrane. Essentially, the binding site is moved from one face of the membrane to the other by changes in the conformation of the carrier protein (Figure.7b and Table.1). Note that a substance transported by carrier-mediated facilitated diffusion, such as glucose, moves down its concentration gradient, just as in simple diffusion. Glucose is normally in higher concentrations in the blood than in the cells, where it is rapidly used for ATP synthesis. So, glucose transport within the body is typically unidirectional into the cells. However, carrier-mediated transport is limited by the number of protein carriers present. For example, when all the glucose carriers are engaged, they are said to be saturated, and glucose transport is occurring at its maximum rate. Channels are transmembrane proteins that serve to transport substances, usually ions or water, through aqueous channels from one side of the membrane to the other (Figure.7c and d). Binding or association sites exist within the channels, and the channels are selective due to pore size and the charges of the amino acids lining the channel. Some channels, the so-called leakage channels, are always open and simply allow ion or water fluxes according to concentration gradients. Others are gated and are controlled (opened or closed) by various chemical or electrical signals. Like carriers, many channels can be inhibited by certain molecules, show saturation, and tend to be specific. Substances moving through them also follow the concentration gradient (always moving down the gradient). When a substance crosses the membrane by simple diffusion, the rate of diffusion is not controllable because the lipid solubility of the membrane is not immediately changeable. By contrast, the rate of facilitated diffusion is controllable because the

10 70 UNIT 1 Organization of the Body (a) Membrane permeable to both solutes and water Solute and water molecules move down their concentration gradients in opposite directions. Fluid volume remains the same in both compartments. Left compartment: Solution with lower osmolarity Right compartment: Solution with greater osmolarity Both solutions have the same osmolarity: volume unchanged H 2 O Solute Membrane Solute molecules (sugar) (b) Membrane permeable to water, impermeable to solutes Solute molecules are prevented from moving but water moves by osmosis. Volume increases in the compartment with the higher osmolarity. Left compartment Right compartment Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move H 2 O Membrane Solute molecules (sugar) Figure.8 Influence of membrane permeability on diffusion and osmosis. permeability of the membrane can be altered by regulating the activity or number of individual carriers or channels. Oxygen, water, glucose, and various ions are vitally important to cellular homeostasis. Their passive transport by diffusion (either simple or facilitated) represents a tremendous saving of cellular energy. Indeed, if these substances had to be transported actively, cell expenditures of ATP would increase exponentially! Osmosis The diffusion of a solvent, such as water, through a selectively permeable membrane is osmosis (oz-mo sis; osmos pushing). Even though water is highly polar, it passes via osmosis through the lipid bilayer (Figure.7d). This is surprising because you d expect water to be repelled by the hydrophobic lipid tails. Although still hypothetical, one explanation is that random movements of the membrane lipids open small gaps between their wiggling tails, allowing water to slip and slide its way through the membrane by moving from gap to gap. Water also moves freely and reversibly through water-specific channels constructed by transmembrane proteins called aquaporins (AQPs). Although aquaporins are believed to be present in all cell types, they are particularly abundant in red blood cells and in cells involved in water balance such as kidney tubule cells. Osmosis occurs whenever the water concentration differs on the two sides of a membrane. If distilled water is present on both sides of a selectively permeable membrane, no net osmosis occurs, even though water molecules move in both directions through the membrane. If the solute concentration on the two sides of

11 Chapter Cells: The Living Units 71 (a) Isotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). (b) Hypertonic solutions Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). (c) Hypotonic solutions Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure.9 The effect of solutions of varying tonicities on living red blood cells. the membrane differs, water concentration differs as well (as solute concentration increases, water concentration decreases). The extent to which water s concentration is decreased by solutes depends on the number, not the type, of solute particles, because one molecule or one ion of solute (theoretically) displaces one water molecule. The total concentration of all solute particles in a solution is referred to as the solution s osmolarity (oz mo-lar ĭ-te). When equal volumes of aqueous solutions of different osmolarity are separated by a membrane that is permeable to all molecules in the system, net diffusion of both solute and water occurs, each moving down its own concentration gradient. Eventually, equilibrium is reached when the water concentration on the left equals that on the right, and the solute concentration on both sides is the same (Figure.8a). If we consider the same system, but make the membrane impermeable to solute molecules, we see quite a different result (Figure.8b). Water quickly diffuses from the left to the right compartment and continues to do so until its concentration is the same on the two sides of the membrane. Notice that in this case equilibrium results from the movement of water alone (the solutes are prevented from moving). Notice also that the movement of water leads to dramatic changes in the volumes of the two compartments. The last example mimics osmosis across plasma membranes of living cells, with one major difference. In our examples, the volumes of the compartments are infinitely expandable and the effect of pressure exerted by the added weight of the higher fluid column is not considered. In living plant cells, which have rigid cell walls external to their plasma membranes, this is not the case. As water diffuses into the cell, the point is finally reached where the hydrostatic pressure (the back pressure exerted by water against the membrane) within the cell is equal to its osmotic pressure (the tendency of water to move into the cell by osmosis). At this point, there is no further (net) water entry. As a rule, the higher the amount of nondiffusible, or nonpenetrating, solutes in a cell, the higher the osmotic pressure and the greater the hydrostatic pressure that must be developed to resist further net water entry. However, such major changes in hydrostatic (and osmotic) pressures do not occur in living animal cells, which lack rigid cell walls. Osmotic imbalances cause animal cells to swell or shrink (due to net water gain or loss) until either the solute concentration is the same on both sides of the plasma membrane, or the membrane is stretched to its breaking point. Such changes in animal cells lead us to the important concept of tonicity (to-nis ĭ-te). As noted, many molecules, particularly intracellular proteins and selected ions, are prevented from diffusing through the plasma membrane. Consequently, any change in their concentration alters the water concentration on the two sides of the membrane and results in a net loss or gain of water by the cell. The ability of a solution to change the shape or tone of cells by altering their internal water volume is called tonicity (tono tension). Solutions with the same concentrations of nonpenetrating solutes as those found in cells (0.9% saline or 5% glucose) are isotonic ( the same tonicity ). Cells exposed to such solutions retain their normal shape, and exhibit no net loss or gain of water (Figure.9a). As you might expect, the body s extracellular fluids and most intravenous solutions (solutions infused into the body via a vein) are isotonic.

12 72 UNIT 1 Organization of the Body TABLE.1 Passive Membrane Transport Processes PROCESS ENERGY SOURCE DESCRIPTION EXAMPLES Diffusion Simple diffusion Kinetic energy Net movement of molecules from an area of their higher concentration to an area of their lower concentration, that is, along their concentration gradient Facilitated diffusion Kinetic energy Same as simple diffusion, but the diffusing substance is attached to a lipid-soluble membrane carrier protein or moves through a membrane channel Osmosis Kinetic energy Simple diffusion of water through a selectively permeable membrane Movement of fats, oxygen, carbon dioxide through the lipid portion of the membrane Movement of glucose and some ions into cells Movement of water into and out of cells directly through the lipid phase of the membrane or via membrane channels (aquaporins) Solutions with a higher concentration of nonpenetrating solutes than seen in the cell (for example, a strong saline solution) are hypertonic. Cells immersed in hypertonic solutions lose water and shrink, or crenate (kre nat) (Figure.9b). Solutions that are more dilute (contain a lower concentration of nonpenetrating solutes) than cells are called hypotonic.cells placed in a hypotonic solution plump up rapidly as water rushes into them (Figure.9c). Distilled water represents the most extreme example of hypotonicity. Because it contains no solutes, water continues to enter cells until they finally burst or lyse. Notice that osmolarity and tonicity are not the same thing. A solution s osmolarity is based solely on its total solute concentration. In contrast, its tonicity is based on how the solution affects cell volume, which depends on (1) solute concentration and (2) solute permeability of the plasma membrane. Osmolarity is expressed as osmoles per liter (osmol/l) where 1 osmol is equal to 1 mole of nonionizing molecules.* A 0.-osmol/L solution of NaCl is isotonic because sodium ions are usually prevented from diffusing through the plasma membrane. But if the cell is immersed in a 0.-osmol/L solution of a penetrating solute, the solute will enter the cell and water will follow. The cell will swell and burst, just as if it had been placed in pure water. Osmosis is extremely important in determining distribution of water in the various fluid-containing compartments of the body (in cells, in blood, and so on). In general, osmosis continues until osmotic and hydrostatic pressures acting at the membrane are equal. For example, water is forced out of capillary blood by the hydrostatic pressure of the blood against the capillary wall, but the presence in blood of solutes that are too large to cross the capillary membrane draws water back into the bloodstream. As a result, very little net loss of plasma fluid occurs. Simple diffusion and osmosis occurring directly through the plasma membrane are not selective processes. In those processes, whether a molecule can pass through the membrane *Osmolarity (Osm) is determined by multiplying molarity (moles per liter, or M) by the number of particles resulting from ionization. For example, since NaCl ionizes to Na Cl,a 1M solution of NaCl is a 2-Osm solution. For substances that do not ionize (e.g., glucose), molarity and osmolarity are the same. depends chiefly on its size or its solubility in lipid, not on its unique structure. Facilitated diffusion, on the other hand, is often highly selective. The carrier for glucose, for example, combines specifically with glucose, in much the same way an enzyme binds to its specific substrate and ion channels allow only selected ions to pass. HOMEOSTATIC IMBALANCE Hypertonic solutions are sometimes infused intravenously into the bloodstream of edematous patients (those swollen because water is retained in their tissues) to draw excess water out of the extracellular space and move it into the bloodstream so that it can be eliminated by the kidneys. Hypotonic solutions may be used (with care) to rehydrate the tissues of extremely dehydrated patients. In less extreme cases of dehydration, drinking hypotonic fluids (colas, apple juice, and sports drinks) usually does the trick. Table.1 summarizes passive membrane transport processes. CHECK YOUR UNDERSTANDING 7. What is the energy source for all types of diffusion? 8. What determines the direction of any diffusion process? 9. What are the two types of facilitated diffusion and how do they differ? For answers, see Appendix G. Active Processes Differentiate between primary and secondary active transport. Compare and contrast endocytosis and exocytosis in terms of function and direction. Compare and contrast pinocytosis, phagocytosis, and receptor-mediated endocytosis.

13 Whenever a cell uses the bond energy of ATP to move solutes across the membrane, the process is referred to as active. Substances moved actively across the plasma membrane are usually unable to pass in the necessary direction by passive transport processes. The substance may be too large to pass through the channels, incapable of dissolving in the lipid bilayer, or unable to move down its concentration gradient. There are two major means of active membrane transport: active transport and vesicular transport. Active Transport Active transport, like carrier-mediated facilitated diffusion, requires carrier proteins that combine specifically and reversibly with the transported substances. However, facilitated diffusion always follows concentration gradients because its driving force is kinetic energy. In contrast, the active transporters or solute pumps move solutes, most importantly ions (such as Na,K, and Ca 2 ), uphill against a concentration gradient. To do this work, cells must expend the energy of ATP. Active transport processes are distinguished according to their source of energy. In primary active transport, the energy to do work comes directly from hydrolysis of ATP.In secondary active transport, transport is driven indirectly by energy stored in ionic gradients created by operation of primary active transport pumps. Secondary active transport systems are all coupled systems; that is, they move more than one substance at a time. If the two transported substances are moved in the same direction, the system is a symport system (sym same). If the transported substances wave to each other as they cross the membrane in opposite directions, the system is an antiport system (anti opposite, against). Let s examine these processes more carefully. Primary Active Transport In primary active transport, hydrolysis of ATP results in the phosphorylation of the transport protein. This step causes the protein to change its shape in such a manner that it pumps the bound solute across the membrane. Primary active transport systems include calcium and hydrogen pumps, but the most investigated example of a primary active transport system is the operation of the sodiumpotassium pump, for which the carrier, or pump, is an enzyme called Na -K ATPase. In the body, the concentration of K inside the cell is some 10 times higher than that outside, and the reverse is true of Na. These ionic concentration differences are essential for excitable cells like muscle and nerve cells to function normally and for all body cells to maintain their normal fluid volume. Because Na and K leak slowly but continuously through leakage channels in the plasma membrane along their concentration gradient (and cross more rapidly in stimulated muscle and nerve cells), the Na -K Chapter Cells: The Living Units 7 pump operates more or less continuously as an antiporter. It simultaneously drives Na out of the cell against a steep concentration gradient and pumps K back in. The electrochemical gradients maintained by the Na -K pump underlie most primary and secondary active transport of nutrients and ions, and are crucial for cardiac and skeletal muscle and neuron function. The step-by-step operation of the Na -K pump is described in Focus on Primary Active Transport: The Na + -K + Pump (Figure.10) on p. 74. Make sure you understand this process thoroughly before moving on to the topic of secondary active transport. Secondary Active Transport A single ATP-powered pump, such as the Na -K pump, can indirectly drive the secondary active transport of several other solutes. By moving sodium across the plasma membrane against its concentration gradient, the pump stores energy (in the ion gradient). Then, just as water pumped uphill can do work as it flows back down (to turn a turbine or water wheel), a substance pumped across a membrane can do work as it leaks back, propelled downhill along its concentration gradient. In this way, as sodium moves back into the cell with the help of a carrier protein (facilitated diffusion), other substances are dragged along, or cotransported, by a common carrier protein (Figure.11). A carrier moving two substances in the same direction is a symport system. For example, some sugars, amino acids, and many ions are cotransported in this way into cells lining the small intestine. Both cotransported substances move passively because the energy for this type of transport is the concentration gradient of the ion (in this case Na ). Na has to be pumped back out into the lumen (cavity) of the intestine to maintain its diffusion gradient. Ion gradients can also be used to drive antiport systems such as those that help to regulate intracellular ph by using the sodium gradient to expel hydrogen ions. Regardless of whether the energy is provided directly (primary active transport) or indirectly (secondary active transport), each membrane pump or cotransporter transports only specific substances. For this reason, active transport systems provide a way for the cell to be very selective in cases where substances cannot pass by diffusion. (No pump no transport.) Vesicular Transport In vesicular transport, fluids containing large particles and macromolecules are transported across cellular membranes inside membranous sacs called vesicles. Vesicular transport processes that eject substances from the cell interior into the extracellular fluid are called exocytosis (ek so-si-to sis; out of the cell ). Those in which the cell ingests small patches of the plasma

14 Figure.10 FOCUS Primary Active Transport: The Na + -K + Pump Primary active transport is the process in which ions are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na + -K + pump is an important example of primary active transport. Extracellular fluid Na + Na + -K + pump ATP-binding site K + Na + bound Cytoplasm 1 Cytoplasmic Na + binds to pump protein. P K + released ATP ADP 6 K + is released from the pump protein and Na + sites are ready to bind Na + again. The cycle repeats. 2 Binding of Na + promotes phosphorylation of the protein by ATP. Na + released K + bound P P i K + 5 K + binding triggers release of the phosphate. Pump protein returns to its original conformation. Phosphorylation causes the protein to change shape, expelling Na + to the outside. P 4 Extracellular K + binds to pump protein. 74

15 Chapter Cells: The Living Units 75 Extracellular fluid Glucose Na + Na + Na+ K + Na + -K + pump Na + -glucose symport transporter loading glucose from ECF Na + Na + -glucose symport transporter releasing glucose into the cytoplasm ATP Na + Cytoplasm 1 The ATP-driven Na + -K + pump 2 As Na + diffuses back across the membrane stores energy by creating a steep concentration gradient for Na + entry into the cell. through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell. (ECF = extracellular fluid) Figure.11 Secondary active transport. membrane and moves substances from the cell exterior to the cell interior are called endocytosis (en do-si-to sis; within the cell ). Vesicular transport is also used for combination processes such as transcytosis, moving substances into, across, and then out of the cell, and substance, or vesicular, trafficking, moving substances from one area (or organelle) in the cell to another. Like solute pumping, vesicular transport processes are energized by ATP (or in some cases another energy-rich compound, GTP guanosine triphosphate). Endocytosis, Transcytosis, and Vesicular Trafficking Virtually all forms of vesicular transport involve an assortment of protein-coated vesicles of three types and, with some exceptions, all are mediated by membrane receptors. Before we get specific about each type of coated vesicular transport, let s look at the general scheme of endocytosis. Protein-coated vesicles provide the main route for endocytosis and transcytosis of bulk solids, most macromolecules, and fluids. On occasion, these vesicles are also hijacked by pathogens seeking entry into a cell. Figure.12 shows the basic steps in endocytosis and transcytosis. 1 The substance to be taken into the cell by endocytosis is progressively enclosed by an infolding portion of the plasma membrane called a coated pit. The coating is most often the bristlelike clathrin (kla thrin; lattice clad ) protein coating found on the cytoplasmic face of the pit. The clathrin coat (clathrin and some accessory proteins) acts both in cargo selection and in deforming the membrane to produce the vesicle. 2 The vesicle detaches, and the coat proteins are recycled back to the plasma membrane. 4 The uncoated vesicle then typically fuses with a processing and sorting vesicle called an endosome. 5 Some membrane components and receptors of the fused vesicle may be recycled back to the plasma membrane in a transport vesicle. 6 The remaining contents of the vesicle may (a) combine with a lysosome (li sō-sōm), a specialized cell structure containing digestive enzymes, where the ingested substance is degraded or released (if iron or cholesterol), or (b) be transported completely across the cell and released by exocytosis on the opposite side (transcytosis). Transcytosis is common in the endothelial cells lining blood vessels because it provides a quick means to get substances from the blood to the interstitial fluid. Based on the nature and quantity of material taken up and the means of uptake, three types of endocytosis that use clathrin-coated vesicles are recognized: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis (fag o-si-to sis; cell eating ) is the type of endocytosis in which the cell engulfs some relatively large or solid material, such as a clump of bacteria, cell debris, or inanimate particles (asbestos fibers or glass, for example) (Figure.1a). When a particle binds to receptors on the cell s surface, cytoplasmic extensions called pseudopods (soo do-pahdz; pseudo false, pod foot) form and flow around the particle and engulf it. The endocytotic vesicle formed in this way is called a phagosome (fag o-sōm; eaten body ). In most cases, the phagosome then fuses with a lysosome and its contents are digested. Any indigestible contents are ejected from the cell by exocytosis. In the human body, only macrophages and certain white blood cells are experts at phagocytosis. Commonly referred to as phagocytes, these cells help police and protect the body by ingesting and disposing of bacteria, other foreign substances, and dead tissue cells. The disposal of dying cells is crucial, because dead cell remnants trigger inflammation in the surrounding area or may stimulate an undesirable immune response. Most phagocytes move about by amoeboid motion (ah-me boyd; changing shape ); that is, the flowing of their cytoplasm into temporary pseudopods allows them to creep along.

16 76 UNIT 1 Organization of the Body 1 Coated pit ingests substance. Extracellular fluid Plasma membrane Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Cytoplasm Coat proteins detach and are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle Endosome 4 Uncoated vesicle fuses with a 5 Transport vesicle sorting vesicle called an endosome. containing membrane components moves to the plasma membrane for recycling. Lysosome (a) 6 Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis). (b) Figure.12 Events of endocytosis mediated by protein-coated pits. Note the three possible fates for a vesicle and its contents, shown in 5 and 6. In pinocytosis ( cell drinking ), also called fluid-phase endocytosis, a bit of infolding plasma membrane (which begins as a clathrin-coated pit) surrounds a very small volume of extracellular fluid containing dissolved molecules (Figure.1b). This droplet enters the cell and fuses with an endosome. Unlike phagocytosis, pinocytosis is a routine activity of most cells, affording them a nonselective way of sampling the extracellular fluid. It is particularly important in cells that absorb nutrients, such as cells that line the intestines. As mentioned, bits of the plasma membrane are removed when the membranous sacs are internalized. However, these membranes are recycled back to the plasma membrane by exocytosis as described shortly, so the surface area of the plasma membrane remains remarkably constant. Receptor-mediated endocytosis is the main mechanism for the specific endocytosis and transcytosis of most macromolecules by body cells, and it is exquisitely selective (Figure.1c). It is also the mechanism that allows cells to concentrate material that is present only in very small amounts in the extracellular fluid. The receptors for this process are plasma membrane proteins that bind only certain substances. Both the receptors and attached molecules are internalized in a clathrin-coated pit and then dealt with in one of the ways discussed above. Substances taken up by receptor-mediated endocytosis include enzymes, insulin (and some other hormones), low-density lipoproteins (such as cholesterol attached to a transport protein), and iron. Unfortunately, flu viruses, diphtheria, and cholera toxins use this route to enter and attack our cells. Other coat proteins are also used for certain types of vesicular transport. Caveolae (ka ve-o le; little caves ), tubular or flaskshaped inpocketings of the plasma membrane seen in many cell types, are involved in a unique kind of receptor-mediated endocytosis called potosis. Like clathrin-coated pits, caveolae capture specific molecules (folic acid, tetanus toxin) from the extracellular fluid in coated vesicles and participate in some forms of transcytosis. However, caveolae are smaller than clathrin-coated

17 Chapter Cells: The Living Units 77 TABLE.2 Active Membrane Transport Processes PROCESS ENERGY SOURCE DESCRIPTION EXAMPLES Active Transport Primary active transport Secondary active transport ATP Ion concentration gradient maintained with ATP Transport of substances against a concentration (or electrochemical) gradient. Performed across the plasma membrane by a solute pump, directly using energy of ATP hydrolysis. Cotransport (coupled transport) of two solutes across the membrane. Energy is supplied indirectly by the ion gradient created by primary active transport. Symporters move the transported substances in the same direction; antiporters move transported substances in opposite directions across the membrane. Ions (Na, K, H, Ca 2, and others) Movement of polar or charged solutes, e.g., amino acids (into cell by symporters); Ca 2+, H + (out of cells via antiporters) Vesicular Transport Exocytosis ATP Secretion or ejection of substances from a cell. The substance is enclosed in a membranous vesicle, which fuses with the plasma membrane and ruptures, releasing the substance to the exterior. Endocytosis Via clathrin-coated vesicles ATP Phagocytosis ATP Cell eating : A large external particle (proteins, bacteria, dead cell debris) is surrounded by a seizing foot and becomes enclosed in a vesicle (phagosome). Pinocytosis (fluid-phase endocytosis) Receptormediated endocytosis Via caveolin-coated vesicles (caveolae) Intracellular vesicular trafficking Via coatomercoated vesicles ATP ATP ATP ATP Plasma membrane sinks beneath an external fluid droplet containing small solutes. Membrane edges fuse, forming a fluid-filled vesicle. Selective endocytosis and transcytosis. External substance binds to membrane receptors. Selective endocytosis (and transcytosis). External substance binds to membrane receptors (often associated with lipid rafts). Vesicles pinch off from organelles and travel to other organelles to deliver their cargo. Secretion of neurotransmitters, hormones, mucus, etc.; ejection of cell wastes In the human body, occurs primarily in protective phagocytes (some white blood cells and macrophages) Occurs in most cells; important for taking in dissolved solutes by absorptive cells of the kidney and intestine Means of intake of some hormones, cholesterol, iron, and most macromolecules Roles not fully known; proposed roles include cholesterol regulation and trafficking, and platforms for signal transduction Accounts for nearly all intracellular trafficking between certain organelles (endoplasmic reticulum and Golgi apparatus). Exceptions include vesicles budding from the trans face of the Golgi apparatus, which are clathrin-coated. vesicles. Additionally, their cage-like protein coat is thinner and composed of a different protein called caveolin. Caveolae are closely associated with lipid rafts that are platforms for G proteins, receptors for hormones (for example, insulin), and enzymes involved in cell regulation. These vesicles appear to provide sites for cell signaling and cross talk between signaling pathways. Their precise role in the cell is still being worked out. Vesicles coated with coatomer (COP1 and COP2) proteins are used in most types of intracellular vesicular trafficking, in which vesicles transport substances between organelles.

18 78 UNIT 1 Organization of the Body Phagosome (a) Phagocytosis The cell engulfs a large particle by forming projecting pseudopods ( false feet ) around it and enclosing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain in the vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be proteincoated but has receptors capable of binding to microorganisms or solid particles. Extracellular fluid Secretory vesicle Plasma membrane SNARE (t-snare) Vesicle SNARE (v-snare) Molecule to be secreted Cytoplasm (a) The process of exocytosis 1 The membranebound vesicle migrates to the plasma membrane. (b) Pinocytosis The cell gulps drops of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated. Fused v- and t-snares 2 There, proteins at the vesicle surface (v-snares) bind with t-snares (plasma membrane proteins). Fusion pore formed Vesicle Receptor recycled to plasma membrane Vesicle (c) Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins in regions of coated pits, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles. The vesicle and plasma membrane fuse and a pore opens up. 4 Vesicle contents are released to the cell exterior. Figure.1 Comparison of three types of endocytosis. Exocytosis The process of exocytosis, typically stimulated by a cell-surface signal such as binding of a hormone to a membrane receptor or a change in membrane voltage, accounts for hormone secretion, neurotransmitter release, mucus secretion, and in some cases, ejection of wastes. The substance to be removed from the cell is first enclosed in a protein-coated membranous sac called a vesicle. In most cases, the vesicle migrates to the plasma membrane, fuses with it, and then ruptures, spilling the sac contents out of the cell (Figure.14). Exocytosis, like other cases in which vesicles are targeted to their destinations, involves a docking process in which transmembrane proteins on the vesicles, fancifully called v-snares (v for vesicle), recognize certain plasma membrane proteins, (b) Photomicrograph of a secretory vesicle releasing its contents by exocytosis (100,000 ) Figure.14 Exocytosis.

19 Chapter Cells: The Living Units 79 + Extracellular fluid K + + K + K + K + K + K + K+ Na + Cytoplasm Cl Na + Cl Potassium leakage channels A Na + K + K + K + K+ K+ K+ K + A + + Protein anion (unable to follow K + through the membrane) 1 K + diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K + results in a negative charge on the inner plasma membrane face. 2 K + also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. A negative membrane potential ( 90 mv) is established when the movement of K + out of the cell equals K + movement into the cell. At this point, the concentration gradient promoting K + exit exactly opposes the electrical gradient for K + entry. Figure.15 The key role of K + in generating the resting membrane potential. The resting membrane potential is largely determined by K + because the membrane is much more permeable to K + than Na + at rest. The active transport of sodium and potassium ions (in a ratio of :2) by the Na + -K + pump maintains these conditions. called t-snares (t for target), and bind with them. This binding causes the membranes to corkscrew together and fuse, rearranging the lipid monolayers without mixing them (Figure.14a). As described, membrane material added by exocytosis is removed by endocytosis the reverse process. Table.2 summarizes active membrane transport processes. CHECK YOUR UNDERSTANDING 10. What happens when the Na + -K + pump is phosphorylated? When K + binds to the pump protein? 11. As a cell grows, its plasma membrane expands. Does this membrane expansion involve endocytosis or exocytosis? 12. Phagocytic cells gather in the lungs, particularly in the lungs of smokers. What is the connection? 1. What vesicular transport process allows a cell to take in cholesterol from the extracellular fluid? For answers, see Appendix G. The Plasma Membrane: Generation of a Resting Membrane Potential Define membrane potential and explain how the resting membrane potential is established and maintained. As you re now aware, the selective permeability of the plasma membrane can lead to dramatic osmotic flows, but that is not its only consequence. An equally important result is the generation of a membrane potential, or voltage, across the membrane. A voltage is electrical potential energy resulting from the separation of oppositely charged particles. In cells, the oppositely charged particles are ions, and the barrier that keeps them apart is the plasma membrane. In their resting state, all body cells exhibit a resting membrane potential that typically ranges from 50 to 100 millivolts (mv), depending on cell type. For this reason, all cells are said to be polarized. The minus sign before the voltage indicates that the inside of the cell is negative compared to its outside. This voltage (or charge separation) exists only at the membrane.ifwe were to add up all the negative and positive charges in the cytoplasm, we would find that the cell interior is electrically neutral. Likewise, the positive and negative charges in the extracellular fluid balance each other exactly. So how does the resting membrane potential come about, and how is it maintained? The short answer is that diffusion causes ionic imbalances that polarize the membrane, and active transport processes maintain that membrane potential. First, let s look at how diffusion polarizes the membrane. Many kinds of ions are found both inside cells and in the extracellular fluid, but the resting membrane potential is determined mainly by the concentration gradient of potassium (K ) and by the differential permeability of the plasma membrane to K and other ions (Figure.15). Recall that K and protein anions predominate inside body cells, and the extracellular fluid contains relatively more Na, which is largely balanced by Cl. The unstimulated plasma membrane is somewhat permeable to K because of leakage channels, but impermeable to the protein anions. Consequently, K diffuses out of the cell along its concentration gradient but the protein anions are unable to follow,

20 80 UNIT 1 Organization of the Body and this loss of positive charges makes the membrane interior more negative (Figure.15). As more and more K leaves the cell, the negativity of the inner membrane face becomes great enough to attract K back toward and even into the cell. At this point ( 90 mv), potassium s concentration gradient is exactly balanced by the electrical gradient (membrane potential), and one K enters the cell as one leaves. In many cells, sodium (Na ) also contributes to the resting membrane potential. Sodium is strongly attracted to the cell interior by its concentration gradient, bringing the resting membrane potential to 70 mv. However, potassium still largely determines the resting membrane potential because the membrane is much more permeable to K than to Na. Even though the membrane is permeable to Cl, in most cells Cl does not contribute to the resting membrane potential, because its entry is resisted by the negative charge of the interior. We may be tempted to believe that massive flows of K ions are needed to generate the resting potential, but this is not the case. Surprisingly, the number of ions producing the membrane potential is so small that it does not change ion concentrations in any significant way. In a cell at rest, very few ions cross its plasma membrane. However, Na and K are not at equilibrium and there is some net movement of K out of the cell and of Na into the cell. Na is strongly pulled into the cell by both its concentration gradient and the interior negative charge. If only passive forces were at work, these ion concentrations would eventually become equal inside and outside the cell. Now let s look at how active transport processes maintain the membrane potential that diffusion has established, with the result that the cell exhibits a steady state. The rate of active transport is equal to, and depends on, the rate of Na diffusion into the cell. If more Na enters, more is pumped out. (This is like being in a leaky boat. The more water that comes in, the faster you bail!) The Na -K pump couples sodium and potassium transport and, on average, each turn of the pump ejects Na out of the cell and carries 2K back in (see Figure.10). Because the membrane is always 50 to 100 times more permeable to K, the ATP-dependent Na -K pump maintains both the membrane potential (the charge separation) and the osmotic balance. Indeed, if Na was not continuously removed from cells, in time so much would accumulate intracellularly that the osmotic gradient would draw water into the cells, causing them to burst. Now that we ve introduced the membrane potential, we can add a detail or two to our discussion of diffusion. Earlier we said that solutes diffuse down their concentration gradient. This is true for uncharged solutes, but only partially true for ions. The negatively and positively charged faces of the plasma membrane can help or hinder diffusion of ions driven by a concentration gradient. It is more correct to say that ions diffuse according to electrochemical gradients, thereby recognizing the effect of both electrical and concentration (chemical) forces. Consequently, although diffusion of K across the plasma membrane is aided by the membrane s greater permeability to it and by the ion s concentration gradient, its diffusion is resisted somewhat by the positive charge on the cell exterior. In contrast, Na is drawn into the cell by a steep electrochemical gradient, and the limiting factor is the membrane s relative impermeability to it. As we will describe in detail in later chapters, upsetting the resting membrane potential by transient opening of Na and K channels in the plasma membrane is a normal means of activating neurons and muscle cells. CHECK YOUR UNDERSTANDING 14. What event or process establishes the resting membrane potential? 15. Is the inside of the plasma membrane negative or positive relative to its outside in a polarized membrane? For answers, see Appendix G. The Plasma Membrane: Cell-Environment Interactions Describe the role of the glycocalyx when cells interact with their environment. List several roles of membrane receptors and that of voltage-sensitive membrane channel proteins. Cells are biological minifactories and, like other factories, they receive and send orders from and to the outside community. But how does a cell interact with its environment, and what activates it to carry out its homeostatic functions? Although cells sometimes interact directly with other cells, this is not always the case. In many cases cells respond to extracellular chemicals, such as hormones and neurotransmitters distributed in body fluids. Cells also interact with extracellular molecules that act as signposts to guide cell migration during development and repair. Whether cells interact directly or indirectly, however, the glycocalyx is always involved. The best understood of the participating glycocalyx molecules fall into two large families cell adhesion molecules and plasma membrane receptors (see Figure.4). Another group of membrane proteins, the voltage-sensitive channel proteins, are important in cells that respond to electrical signals. Roles of Cell Adhesion Molecules Thousands of cell adhesion molecules (CAMs) are found on almost every cell in the body. They play key roles in embryonic development and wound repair (situations where cell mobility is important) and in immunity. These sticky glycoproteins (cadherins and integrins) act as 1. The molecular Velcro that cells use to anchor themselves to molecules in the extracellular space and to each other (see desmosome discussion on pp ) 2. The arms that migrating cells use to haul themselves past one another. SOS signals sticking out from the blood vessel lining that rally protective white blood cells to a nearby infected or injured area

21 4. Mechanical sensors that respond to local tension at the cell surface by stimulating synthesis or degradation of adhesive membrane junctions 5. Transmitters of intracellular signals that direct cell migration, proliferation, and specialization Roles of Membrane Receptors A huge and diverse group of integral proteins and glycoproteins that serve as binding sites are collectively known as membrane receptors. Some function in contact signaling, and others in chemical signaling. Let s take a look. Contact Signaling Contact signaling is the actual coming together and touching of cells, and it is the means by which cells recognize one another. It is particularly important for normal development and immunity. Some bacteria and other infectious agents use contact signaling to identify their preferred target tissues or organs. Chemical Signaling Most plasma membrane receptors are involved in chemical signaling, and this group will receive the bulk of our attention. Signaling chemicals that bind specifically to plasma membrane receptors are called ligands. Among these ligands are most neurotransmitters (nervous system signals), hormones (endocrine system signals), and paracrines (chemicals that act locally and are rapidly destroyed). Different cells respond in different ways to the same ligand. Acetylcholine, for instance, stimulates skeletal muscle cells to contract, but inhibits heart muscle. Why do different cells have such different responses? The reason is that a target cell s response depends on the internal machinery that the receptor is linked to, not the specific ligand that binds to it. Though cell responses to receptor binding vary widely, there is a fundamental similarity. When a ligand binds to a membrane receptor, the receptor s structure changes, and cell proteins are altered in some way. For example, muscle proteins change shape to generate force. Some membrane receptor proteins are catalytic proteins that function as enzymes. Others, such as the chemically gated channel-linked receptors common in muscle and nerve cells, respond to ligands by transiently opening or closing ion gates, which in turn changes the excitability of the cell. Still other receptors are coupled to enzymes or ion channels by a regulatory molecule called a G protein. Because nearly every cell in the body displays at least some of these receptors, we will spend a bit more time with them. The lipid rafts mentioned earlier group together many receptor-mediated elements, thus facilitating cell signaling. G protein linked receptors exert their effect indirectly through a G protein, which acts as a middleman or relay to activate (or inactivate) a membrane-bound enzyme or ion channel. As a result, one or more intracellular chemical signals, commonly called second messengers, are generated and connect plasma membrane events to the internal metabolic machinery of the cell. Two very important second messengers are cyclic AMP and ionic calcium, both of which typically activate protein kinase enzymes, which transfer phosphate groups from Chapter Cells: The Living Units 81 ATP to other proteins. In this way, the protein kinases can activate a whole series of enzymes that bring about the desired cellular activity. Because a single enzyme can catalyze hundreds of reactions, the amplification effect of such a chain of events is tremendous. Focus on G Proteins (Figure.16) on p. 82 describes how a G protein signaling system works. Take a moment to study this carefully because this key signaling pathway is involved in neurotransmission, smell, vision, and hormone action (Chapters 11, 15, and 16). One important signaling molecule must be mentioned even though it doesn t act in any of the ways we have already described. Nitric oxide (NO), one of nature s simplest molecules, is made of a single atom of nitrogen and one of oxygen. It is also an environmental pollutant and the first gas known to act as a biological messenger. Because of its tiny size, it slips into and out of cells easily. Its unpaired electron makes it highly reactive and it reacts with head-spinning speed with other key molecules to spur cells into a broad array of activities. You will be hearing more about NO later (in the neural, cardiovascular, and immune system chapters). Role of Voltage-Sensitive Membrane Channel Proteins Electrical Signaling In the process known as electrical signaling, certain plasma membrane proteins are channel proteins that respond to changes in membrane potential by opening or closing the channel. Such voltage-gated channels are common in excitable tissues like neural and muscle tissues, and are indispensable to their normal functioning. CHECK YOUR UNDERSTANDING 16. What term is used to indicate signaling chemicals that bind to membrane receptors? Which type of membrane receptor is most important in directing intracellular events by promoting formation of second messengers? For answer, see Appendix G. The Cytoplasm Describe the composition of the cytosol. Define inclusions and list several types. Discuss the structure and function of mitochondria. Discuss the structure and function of ribosomes, the endoplasmic reticulum, and the Golgi apparatus, including functional interrelationships among these organelles. Compare the functions of lysosomes and peroxisomes. Cytoplasm ( cell-forming material ) is the cellular material between the plasma membrane and the nucleus. It is the site where most cellular activities are accomplished. Although early microscopists thought that the cytoplasm was a structureless gel, the electron microscope reveals that it consists of three major elements: the cytosol, organelles, and inclusions.

22 Figure.16 FOCUS G Proteins G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. The sequence described here is like a molecular relay race. Instead of a baton passed from runner to runner, the message is passed from molecule to molecule as it makes its way across the cell membrane from outside to inside the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger 1 Ligand (1st messenger) 2 The activated receptor binds to a G Activated G protein activates binds to the receptor. The receptor is activated and changes shape. protein and activates it. During activation, a G protein changes shape (turns on ) causing it to release GDP and bind GTP. (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change. Extracellular fluid Effector protein (e.g., an enzyme) Ligand Receptor G protein GDP GTP GTP Inactive 2nd messenger Active 2nd messenger GTP 4 Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2.) 5 Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. Activated kinase enzymes Cascade of cellular responses (metabolic and structural changes) 6 Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various cell responses. Intracellular fluid 82

23 The cytosol (si to-sol) is the viscous, semitransparent fluid in which the other cytoplasmic elements are suspended. It is a complex mixture with properties of both a colloid and a true solution. Dissolved in the cytosol, which is largely water, are proteins, salts, sugars, and a variety of other solutes. The cytoplasmic organelles are the metabolic machinery of the cell. Each type of organelle carries out a specific function for the cell some synthesize proteins, others package those proteins, and so on. Inclusions are chemical substances that may or may not be present, depending on cell type. Examples include stored nutrients, such as the glycogen granules abundant in liver and muscle cells; lipid droplets common in fat cells; pigment (melanin) granules seen in certain cells of skin and hair; water-containing vacuoles; and crystals of various types. Cytoplasmic Organelles The cytoplasmic organelles ( little organs ) are specialized cellular compartments, each performing its own job to maintain the life of the cell. Some organelles, the nonmembranous organelles, lack membranes. Examples are the cytoskeleton, centrioles, and ribosomes. Most organelles, however, are bounded by a membrane similar in composition to the plasma membrane. This membrane enables such membranous organelles to maintain an internal environment different from that of the surrounding cytosol. This compartmentalization is crucial to cell functioning. Without it, thousands of enzymes would be randomly mixed and biochemical activity would be chaotic. The cell s membranous organelles include the mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus. Besides providing splendid isolation, an organelle s membrane often unites it with the rest of an interactive intracellular system called the endomembrane system (see p. 87) and the lipid makeup of its membrane allows it to recognize and interact with other organelles. Now, let us consider what goes on in each of the workshops of our cellular factory. Mitochondria Mitochondria (mi to-kon dre-ah) are threadlike (mitos thread) or lozenge-shaped membranous organelles. In living cells they squirm, elongate, and change shape almost continuously. They are the power plants of a cell, providing most of its ATP supply. The density of mitochondria in a particular cell reflects that cell s energy requirements, and mitochondria are generally clustered where the action is. Busy cells like kidney and liver cells have hundreds of mitochondria, whereas relatively inactive cells (such as unchallenged lymphocytes) have just a few. Mitochondria are enclosed by two membranes, each with the general structure of the plasma membrane (Figure.17). The outer membrane is smooth and featureless, but the inner membrane folds inward, forming shelflike cristae (krĭ ste; crests ) that protrude into the matrix, the gel-like substance within the mitochondrion. Intermediate products of food fuels (glucose and others) are broken down to water and carbon dioxide by Mitochondrial DNA (a) (b) Ribosome Chapter Cells: The Living Units 8 Enzymes Outer mitochondrial membrane Inner mitochondrial membrane Cristae Matrix Figure.17 Mitochondrion. (a) Diagrammatic view of a longitudinally sectioned mitochondrion. (b) Close-up view of a crista showing enzymes (stalked particles). (c) Electron micrograph of a mitochondrion (50,000 ). teams of enzymes, some dissolved in the mitochondrial matrix and others forming part of the crista membrane. As the metabolites are broken down and oxidized, some of the energy released is captured and used to attach phosphate groups to ADP molecules to form ATP. This multistep mitochondrial process (described in Chapter 24) is generally referred to as aerobic cellular respiration (a-er-o bik) because it requires oxygen. Mitochondria are complex organelles: They contain their own DNA, RNA, and ribosomes and are able to reproduce themselves. Mitochondrial genes (some 7 of them) direct the synthesis of 1% of the proteins required for mitochondrial function, and the DNA of the cell s nucleus encodes the remaining proteins needed to carry out cellular respiration. When cellular requirements for ATP increase, the mitochondria synthesize more cristae or simply pinch in half (a process called fission) to increase their number, then grow to their former size. Intriguingly, mitochondria are similar to a specific group of bacteria (the purple bacteria phylum), and mitochondrial DNA (c)

24 84 UNIT 1 Organization of the Body Smooth ER Nuclear envelope Rough ER Ribosomes (a) Diagrammatic view of smooth and rough ER (b) Electron micrograph of smooth and rough ER (10,000 ) Figure.18 The endoplasmic reticulum. is bacteria-like. It is widely believed that mitochondria arose from bacteria that invaded the ancient ancestors of plant and animal cells, and that this unique merger gave rise to all complex cells. Ribosomes Ribosomes (ri bo-sōmz) are small, dark-staining granules composed of proteins and a variety of RNAs called ribosomal RNAs. Each ribosome has two globular subunits that fit together like the body and cap of an acorn. Ribosomes are sites of protein synthesis, a function we discuss in detail later in this chapter. Some ribosomes float freely in the cytoplasm. Others are attached to membranes, forming a complex called the rough endoplasmic reticulum (Figure.18). These two ribosomal populations appear to divide the chore of protein synthesis. Free ribosomes make soluble proteins that function in the cytosol, as well as those imported into mitochondria and some other organelles. Membrane-bound ribosomes synthesize proteins destined either for incorporation into cell membranes or for export from the cell. Ribosomes can switch back and forth between these two functions, attaching to and detaching from the membranes of the endoplasmic reticulum, according to the type of protein they are making at a given time. Endoplasmic Reticulum The endoplasmic reticulum (ER) (en do-plaz mik re-tik u-lum; network within the cytoplasm ) is an extensive system of interconnected tubes and parallel membranes enclosing fluid-filled cavities, or cisternae (sis-ter ne). Coiling and twisting through the cytosol, the ER is continuous with the nuclear membrane and accounts for about half of the cell s membranes. There are two distinct varieties of ER: rough ER and smooth ER. Rough Endoplasmic Reticulum The external surface of the rough ER is studded with ribosomes (Figure.18a, b). Proteins assembled on these ribosomes thread their way into the fluid-filled interior of the ER cisternae, where various fates await them (as described on p. 106). When complete, the newly made proteins are enclosed in coatomer-coated vesicles for their journey to the Golgi apparatus where they undergo further processing. The rough ER has several functions. Its ribosomes manufacture all proteins secreted from cells. For this reason, the rough ER is particularly abundant and well developed in most secretory cells, antibody-producing plasma cells, and liver cells, which produce most blood proteins. It is also the cell s membrane factory because integral proteins and phospholipids that form part of all cellular membranes are manufactured there. The enzymes needed to catalyze lipid synthesis have their active sites on the external (cytosolic) face of the ER membrane, where the needed substrates are readily available. Smooth Endoplasmic Reticulum The smooth ER (see Figures.2 and.18a, b) is continuous with the rough ER and consists of tubules arranged in a looping network. Its enzymes (all integral proteins forming part of its membranes) play no role in

25 Chapter Cells: The Living Units 85 Transport vesicle from rough ER Cis face receiving side of Golgi apparatus Cisternae New vesicles forming New vesicles forming Transport vesicle from trans face Secretory vesicle Trans face shipping side of Golgi apparatus (a) Many vesicles in the process of pinching off from the membranous Golgi apparatus. Golgi apparatus Transport vesicle from the Golgi apparatus (b) Electron micrograph of the Golgi apparatus (90,000 ) Figure.19 Golgi apparatus. Note: In (a), the various and abundant vesicles shown in the process of pinching off from the membranous Golgi apparatus would have a protein coating on their external surfaces. These proteins are omitted from the diagram for simplicity. protein synthesis. Instead, they catalyze reactions involved with the following processes: 1. Lipid metabolism, cholesterol synthesis, and synthesis of the lipid components of lipoproteins (in liver cells) 2. Synthesis of steroid-based hormones such as sex hormones (testosterone-synthesizing cells of the testes are full of smooth ER). Absorption, synthesis, and transport of fats (in intestinal cells) 4. Detoxification of drugs, certain pesticides, and carcinogens (in liver and kidneys) 5. Breakdown of stored glycogen to form free glucose (in liver cells especially) Additionally, skeletal and cardiac muscle cells have an elaborate smooth ER (called the sarcoplasmic reticulum) that plays an important role in calcium ion storage and release during muscle contraction. Except for the examples given above, most body cells contain little, if any, true smooth ER. Golgi Apparatus The Golgi apparatus (gol je) consists of stacked and flattened membranous sacs, shaped like hollow dinner plates, associated with swarms of tiny membranous vesicles (Figure.19). The Golgi apparatus is the principal traffic director for cellular proteins. Its major function is to modify, concentrate, and package the proteins and lipids made at the rough ER. The transport vesicles that bud off from the rough ER move to and fuse with the membranes at the convex cis face, the receiving side, of the Golgi apparatus. Inside the apparatus, the proteins are modified: Some sugar groups are trimmed while others are added, and in some cases, phosphate groups are added. The various proteins are tagged for delivery to a specific address, sorted, and packaged in at least three types of vesicles that bud from the concave trans face (the shipping side) of the Golgi stack. Vesicles containing proteins destined for export pinch off from the trans face as secretory vesicles,or granules, which migrate to the plasma membrane and discharge their contents from the cell by exocytosis (pathway A, Figure.20). Specialized secretory cells, such as the enzyme-producing cells of the pancreas, have a very prominent Golgi apparatus. Besides its packaging-for-release function, the Golgi apparatus pinches off vesicles containing lipids and transmembrane proteins destined for a home in the plasma membrane (pathway B, Figure.20) or other membranous organelles. It also packages digestive

26 86 UNIT 1 Organization of the Body 1 Protein-containing vesicles pinch off rough ER and migrate to fuse with membranes of Golgi apparatus. 2 Proteins are modified within the Golgi compartments. Rough ER ER membrane Phagosome Proteins in cisterna Vesicle becomes lysosome Pathway C: Lysosome containing acid hydrolase enzymes Plasma membrane Proteins are then packaged within different vesicle types, depending on their ultimate destination. Golgi apparatus Pathway A: Vesicle contents destined for exocytosis Secretory vesicle Secretion by exocytosis Extracellular fluid Pathway B: Vesicle membrane to be incorporated into plasma membrane Figure.20 The sequence of events from protein synthesis on the rough ER to the final distribution of those proteins. The protein coats on the transport vesicles are not illustrated. enzymes into membranous lysosomes that remain in the cell (pathway C in Figure.20, and discussed next). Lysosomes Born as endosomes which contain inactive enzymes, lysosomes ( disintegrator bodies ) are spherical membranous organelles containing activated digestive enzymes (Figure.21).As you might guess, lysosomes are large and abundant within phagocytes, the cells that dispose of invading bacteria and cell debris. Lysosomal enzymes can digest almost all kinds of biological molecules. They work best in acidic conditions and so are called acid hydrolases. The lysosomal membrane is adapted to serve lysosomal functions in two important ways. First, it contains H (proton) pumps, which are ATPases that gather hydrogen ions from the surrounding cytosol to maintain the organelle s acidic ph. Second, it retains the dangerous acid hydrolases while permitting the final products of digestion to escape so that they can be used by the cell or excreted. In this way, lysosomes provide sites where digestion can proceed safely within a cell. Lysosomes function as a cell s demolition crew by 1. Digesting particles taken in by endocytosis, particularly ingested bacteria, viruses, and toxins 2. Degrading worn-out or nonfunctional organelles. Performing metabolic functions, such as glycogen breakdown and release 4. Breaking down nonuseful tissues, such as the webs between the fingers and toes of a developing fetus and the uterine lining during menstruation 5. Breaking down bone to release calcium ions into the blood Lysosomes Figure.21 Electron micrograph of a cell containing lysosomes (12,000 ). Light areas are regions where materials are being digested.

27 Chapter Cells: The Living Units 87 The lysosomal membrane is ordinarily quite stable, but it becomes fragile when the cell is injured or deprived of oxygen and when excessive amounts of vitamin A are present. When lysosomes rupture, the cell digests itself, a process called autolysis (aw tol ĭ-sis). Autolysis is the basis for desirable destruction of cells, as in the fourth point listed above. Smooth ER Nucleus Nuclear envelope HOMEOSTATIC IMBALANCE Glycogen and certain lipids in the brain are degraded by lysosomes at a relatively constant rate. In Tay-Sachs disease, an inherited condition seen mostly in Jews from Central Europe, the lysosomes lack an enzyme needed to break down a glycolipid abundant in nerve cell membranes. As a result, the nerve cell lysosomes swell with the undigested lipids, which interfere with nervous system functioning. Affected infants typically have doll-like features and pink translucent skin. At to 6 months of age, the first signs of disease appear (listlessness, motor weakness). These symptoms progress to mental retardation, seizures, blindness, and ultimately death within 18 months. Summary of Interactions in the Endomembrane System The endomembrane system is a system of organelles (most described above) that work together mainly (1) to produce, store, and export biological molecules, and (2) to degrade potentially harmful substances (Figure.22). It includes the ER, Golgi apparatus, secretory vesicles, and lysosomes, as well as the nuclear membrane that is, all of the membranous organelles or elements that are either structurally continuous or arise via forming or fusing transport vesicles. There are continuities between the nuclear envelope (itself an extension of the rough ER) and the rough and smooth ER (Figure.18). The plasma membrane, though not actually an endomembrane, is also functionally part of this system. Besides these direct structural relationships, a wide variety of indirect interactions (indicated by arrows in Figure.22) occur among the members of the system. Some of the vesicles born in the ER migrate to and fuse with the Golgi apparatus or the plasma membrane, and vesicles arising from the Golgi apparatus can become part of the plasma membrane, secretory vesicles, or lysosomes. Peroxisomes Peroxisomes (pĕ-roks ĭ-sōmz; peroxide bodies ) are membranous sacs containing a variety of powerful enzymes, the most important of which are oxidases and catalases. Oxidases use molecular oxygen (O 2 ) to detoxify harmful substances, including alcohol and formaldehyde. Their most important function is to neutralize dangerous free radicals, highly reactive chemicals with unpaired electrons that can scramble the structure of biological molecules. Oxidases convert free radicals to hydrogen peroxide, which is also reactive and dangerous but is quickly converted to water by catalase enzymes. Free radicals and hydrogen peroxide are normal by-products of cellular metabolism, but they have devastating effects on cells if allowed to accumulate. Peroxisomes are especially Vesicle Plasma membrane Lysosome Figure.22 The endomembrane system. numerous in liver and kidney cells, which are very active in detoxification. A significant amount of fatty acid oxidation also occurs in peroxisomes. Hence, they also play a role in energy metabolism. Peroxisomes look like small lysosomes (see Figure.2), and for many years it was thought that they were self-replicating organelles formed when existing peroxisomes simply pinch in half. Recent evidence, however, suggests that most new peroxisomes form by budding off of the endoplasmic reticulum. CHECK YOUR UNDERSTANDING 17. What organelle is the major site of ATP synthesis? 18. What are three organelles involved in protein synthesis and how do these organelles interact in that process? 19. How does the function of lysosomes compare to that of peroxisomes? For answers, see Appendix G. Golgi apparatus Transport vesicle Rough ER Cytoskeleton Name and describe the structure and function of cytoskeletal elements. The cytoskeleton, literally, cell skeleton, is an elaborate network of rods running through the cytosol. It acts as a cell s bones, muscles, and ligaments by supporting cellular structures and providing the machinery to generate various cell movements. The three types of rods in order of increasing size in the cytoskeleton are microfilaments, intermediate filaments, and microtubules. None of these is membrane covered.

28 88 UNIT 1 Organization of the Body (a) Microfilaments Strands made of spherical protein subunits called actins Actin subunit (b) Intermediate filaments Tough, insoluble protein fibers constructed like woven ropes Fibrous subunits (c) Microtubules Hollow tubes of spherical protein subunits called tubulins Tubulin subunits 7 nm 10 nm 25 nm Microfilaments form the blue network surrounding the pink nucleus in this photo. Intermediate filaments form the purple batlike network in this photo. Microtubules appear as gold networks surrounding the cells pink nuclei in this photo. Figure.2 Cytoskeletal elements support the cell and help to generate movement. Diagrammatic views (above) and photos (below). The photos are of fibroblasts treated to fluorescently tag the structure of interest. Microtubules (mi kro-tu būlz), the elements with the largest diameter, are hollow tubes made of spherical protein subunits called tubulins (Figure.2c). Most microtubules radiate from a small region of cytoplasm near the nucleus called the centrosome or cell center (see Figures.2,.25). Microtubules are remarkably dynamic organelles, constantly growing out from the centrosome, disassembling, and then reassembling at the same or different sites. The stiff but bendable microtubules determine the overall shape of the cell, as well as the distribution of cellular organelles. Mitochondria, lysosomes, and secretory vesicles attach to the microtubules like ornaments hanging from tree branches. They are continually moved along the microtubules and repositioned by tiny protein machines called motor proteins (kinesins, dyneins, and others) or motor molecules. The various motor proteins work by changing their shapes. Powered by ATP, some motor proteins appear to act like train engines moving substances along on the microtubular railroad tracks. No one analogy completely captures their action, but the most common analogy has these motor molecules moving hand over hand somewhat like an orangutan gripping, releasing, and then gripping again at a new site further along the microtubule (Figure.24a). Microfilaments (mi kro-fil ah-ments), the thinnest elements of the cytoskeleton, are strands of the protein actin ( ray ) (Figure.2a). Each cell has its own unique arrangement of microfilaments, so no two cells are alike. However, nearly all cells have a fairly dense cross-linked network of microfilaments, called the terminal web, attached to the cytoplasmic side of their plasma membrane. The web strengthens the cell surface and resists compression. Most microfilaments are involved in cell motility or changes in cell shape. You could say that cells move when they get their act(in) together. For example, actin filaments interact with another protein, unconventional myosin (mi o-sin), to generate contractile forces in a cell (Figure.24b). This interaction also forms the cleavage furrow that pinches one cell into two during cell division. Microfilaments attached to CAMs (see Figure.4e) are responsible for the crawling movements of amoeboid motion, and for the membrane changes that accompany endocytosis and exocytosis. Except in muscle cells, where they are highly developed, stable, and long-lived, actin filaments are constantly breaking down and re-forming from smaller subunits whenever and wherever their services are needed. Intermediate filaments are tough, insoluble protein fibers that have a diameter between those of microfilaments and microtubules (Figure.2b). Constructed like woven ropes, intermediate filaments are the most stable and permanent of the cytoskeletal elements and have high tensile strength. Unlike microtubules and microfilaments, intermediate filaments do not bind ATP or serve as tracks for molecular motors to transport intracellular substances. Instead, they attach to desmosomes,

29 Chapter Cells: The Living Units 89 Vesicle ATP Receptor for motor molecule Motor molecule (ATP powered) Centrosome matrix Centrioles Microtubule of cytoskeleton (a) Motor molecules can attach to receptors on vesicles or organelles, and walk the organelles along the microtubules of the cytoskeleton. ATP Motor molecule (ATP powered) (a) Microtubules Cytoskeletal elements (microtubules or microfilaments) (b) In some types of cell motility, motor molecules attached to one element of the cytoskeleton can cause it to slide over another element, as in muscle contraction and cilia movement. Figure.24 Microtubules and microfilaments function in cell motility by interacting with motor molecules. The various motor molecules, all powered by ATP, work by changing their shapes, moving back or forth on something like microscopic legs. With each cycle of shape changes, the motor molecule releases its free end and grips at a site farther along the microtubule or microfilament. and their main job is to act as internal guy-wires to resist pulling forces exerted on the cell. Because the protein composition of intermediate filaments varies in different cell types, there are numerous names for these cytoskeletal elements for example, they are called neurofilaments in nerve cells and keratin filaments in epithelial cells. Centrosome and Centrioles Describe the roles of centrioles in mitosis and in formation of cilia and flagella. As mentioned, microtubules are anchored at one end in an inconspicuous region near the nucleus called the centrosome or cell center. The centrosome acts as a microtubule organizing center. It has few distinguishing marks other than a granular-looking (b) Figure.25 Centrioles. (a) Three-dimensional view of a centriole pair oriented at right angles, as they are usually seen in the cell. The centrioles are located in an inconspicuous region to one side of the nucleus called the centrosome, or cell center. (b) An electron micrograph showing a cross section of a centriole (120,000 ). Notice that it is composed of nine microtubule triplets. matrix that contains paired centrioles, small, barrel-shaped organelles oriented at right angles to each other (Figure.25).The centrosome matrix is best known for generating microtubules and organizing the mitotic spindle in cell division (see Figure.). Each centriole consists of a pinwheel array of nine triplets of microtubules, each connected to the next by nontubulin proteins and arranged to form a hollow tube. Centrioles also form the bases of cilia and flagella, our next topics.

30 90 UNIT 1 Organization of the Body Outer microtubule doublet Dynein arms Central microtubule The doublets also have attached motor proteins, the dynein arms. Cross-linking proteins inside outer doublets Microtubules TEM A cross section through the cilium shows the arrangement of microtubules. Radial spoke Cross-linking proteins inside outer doublets The outer microtubule doublets and the two central microtubules are held together by cross-linking proteins and radial spokes. Radial spoke Plasma membrane Plasma membrane Cilium Basal body Triplet TEM A longitudinal section of a cilium shows microtubules running the length of the structure. TEM A cross section through the basal body. The nine outer doublets of a cilium extend into a basal body where each doublet joins another microtubule to form a ring of nine triplets. Basal body (centriole) Figure.26 Structure of a cilium. (TEM transmission electron micrograph.) Cellular Extensions Describe how the two main types of cell extensions, cilia and microvilli, differ in structure and function. Cilia and Flagella Cilia (sil e-ah; eyelashes ) are whiplike, motile cellular extensions that occur, typically in large numbers, on the exposed surfaces of certain cells (Figure.26). Ciliary action moves substances in one direction across cell surfaces. For example, ciliated cells that line the respiratory tract propel mucus laden with dust particles and bacteria upward away from the lungs. When a cell is about to form cilia, the centrioles multiply and line up beneath the plasma membrane at the free cell surface. Microtubules then begin to sprout from each centriole, forming the ciliary projections by exerting pressure on the plasma membrane. When the projections formed by centrioles are substantially longer, they are called flagella (flah-jel ah). The single example of a flagellated cell in the human body is a sperm, which has one propulsive flagellum, commonly called a tail. Notice that cilia propel other substances across a cell s surface, whereas a flagellum propels the cell itself. Centrioles forming the bases of cilia and flagella are commonly referred to as basal bodies (ba sal) (Figure.26). They were given a separate name because they were thought to be different from the structures seen in the centrosome. We now know that centrioles and basal bodies are modifications of the same structure. The 9 2 pattern of microtubules in the cilium or flagellum itself (nine doublets, or pairs, of microtubules encircling one central pair) differs slightly from that of a centriole (nine microtubule triplets). Additionally, the cilium has flexible wagon wheels of cross-linking proteins (purple in Figure.26), and motor proteins (green dynein arms in Figure.26) that promote movement of the cilium or flagellum. Just how ciliary activity is coordinated is not fully understood, but microtubules are definitely involved. Extending from the microtubule doublets are arms composed of the motor protein dynein (Figure.26). The dynein side arms of one doublet grip the adjacent doublet, and powered by ATP, push it up, release, and then grip again. Because the doublets are physically restricted by other proteins, they cannot slide far and instead are forced to bend. The collective bending action of all the doublets causes the cilium to bend. As a cilium moves, it alternates rhythmically between a propulsive power stroke, when it is nearly straight and moves in an arc, and a recovery stroke, when it bends and returns to its initial position (Figure.27a). With these two strokes, the cilium produces a pushing motion in a single direction that is repeated some 10 to 20 times per second. The activity of cilia in a particular region is coordinated so that the bending of one cilium is

31 Chapter Cells: The Living Units 91 Power, or propulsive, stroke Recovery stroke, when cilium is returning to its initial position Microvillus (a) Phases of ciliary motion. Layer of mucus Figure.28 Microvilli. Actin filaments Terminal web 22. The major function of cilia is to move substances across the free cell surface. What is the major role of microvilli? Cell surface For answers, see Appendix G. (b) Traveling wave created by the activity of many cilia acting together propels mucus across cell surfaces. Figure.27 Ciliary function. quickly followed by the bending of the next and then the next, creating a current at the cell surface that brings to mind the traveling waves that pass across a field of grass on a windy day (Figure.27b). Microvilli Microvilli (mi kro-vil i; little shaggy hairs ) are minute, fingerlike extensions of the plasma membrane that project from a free, or exposed, cell surface (Figure.5 top and Figure.28). They increase the plasma membrane surface area tremendously and are most often found on the surface of absorptive cells such as intestinal and kidney tubule cells. Microvilli have a core of bundled actin filaments that extend into the so-called terminal web of the cytoskeleton of the cell. Actin is sometimes a contractile protein, but in microvilli it appears to function as a mechanical stiffener. CHECK YOUR UNDERSTANDING 20. How are microtubules and microfilaments related functionally? 21. Of microfilaments, microtubules, or intermediate filaments, which is most important in maintaining cell shape? The Nucleus Outline the structure and function of the nuclear envelope, nucleolus, and chromatin. Anything that works, works best when it is controlled. For cells, the control center is the gene-containing nucleus (nucle pit, kernel). The nucleus can be compared to a computer, design department, construction boss, and board of directors all rolled into one. As the genetic library, it contains the instructions needed to build nearly all the body s proteins. Additionally, it dictates the kinds and amounts of proteins to be synthesized at any one time in response to signals acting on the cell. Most cells have only one nucleus, but some, including skeletal muscle cells, bone destruction cells, and some liver cells, are multinucleate (mul tĭ-nu kle-āt), that is, they have many nuclei. The presence of more than one nucleus usually signifies that a larger-than-usual cytoplasmic mass must be regulated. With one exception, all of our body cells are nucleated. The exception is mature red blood cells, whose nuclei are ejected before the cells enter the bloodstream. These anucleate (a-nu kle-āt; a without) cells cannot reproduce and therefore live in the bloodstream for only three to four months before they begin to deteriorate. Without a nucleus, a cell cannot produce mrna to make proteins, and when its enzymes and cell structures start to break down (as all eventually do), they cannot be replaced. The nucleus, averaging 5 µm in diameter, is larger than any of the cytoplasmic organelles. Although most often spherical or oval, its shape usually conforms to the shape of the cell. The nucleus has three recognizable regions or structures: the nuclear envelope (membrane), nucleoli, and chromatin (Figure.29a). Besides these structures, there are several distinct compartments (a splicing factor compartment and others) rich in specific sets of proteins. These are not limited by membranes and

32 92 UNIT 1 Organization of the Body Surface of nuclear envelope. Fracture line of outer membrane Nuclear pores Nuclear envelope Nucleus Chromatin (condensed) Nucleolus Nuclear pore complexes. Each pore is ringed by protein particles. Cisternae of rough ER (a) Figure.29 The nucleus. (a) Three-dimensional diagrammatic view of the nucleus, showing the continuity of its double membrane with the ER. (b) Freeze-fracture transmission electron micrographs (TEMs). Nuclear lamina. The netlike lamina composed of intermediate filaments formed by lamins lines the inner surface of the nuclear envelope. (b) are in a constant state of dynamic flux. Much remains to be learned about them. The Nuclear Envelope The nucleus is bounded by the nuclear envelope, a double membrane barrier separated by a fluid-filled space (similar to the mitochondrial membrane). The outer nuclear membrane is continuous with the rough ER of the cytoplasm and is studded with ribosomes on its external face. The inner nuclear membrane is lined by the nuclear lamina, a network of lamins (rodshaped proteins that assemble to form intermediate filaments) that maintains the shape of the nucleus and acts as a scaffold to organize DNA in the nucleus (Figure.29b, bottom). At various points, the two layers of the nuclear envelope interconnect to form the edges of nuclear pores. An intricate complex of proteins, called a nuclear pore complex (NPC), lines each pore, forming an aqueous transport channel and regulating entry and exit of molecules (e.g., mrnas) and large particles into and out of the nucleus (see Figure.29b, middle). Like other cell membranes, the nuclear envelope is selectively permeable, but here passage of substances is much freer than

33 elsewhere. Small molecules pass through the relatively large nuclear pore complexes unimpeded. Protein molecules imported from the cytoplasm and RNA molecules exported from the nucleus are transported through the central channel of the pores in an energy-dependent process by soluble transport proteins (importins and others). The nuclear envelope encloses a jellylike fluid called nucleoplasm (nu kle-o-plazm) in which other nuclear elements are suspended. Like the cytosol, the nucleoplasm contains dissolved salts, nutrients, and other essential solutes. Nucleoli Nucleoli (nu-kle o-li; little nuclei ) are the dark-staining spherical bodies found within the nucleus. They are not membrane bounded. Typically, there are one or two nucleoli per nucleus, but there may be more. Because they are sites where ribosomal subunits are assembled, nucleoli are usually large in growing cells that are making large amounts of tissue proteins. Nucleoli are associated with nucleolar organizer regions, which contain the DNA that issues genetic instructions for synthesizing ribosomal RNA (rrna). As molecules of rrna are synthesized, they are combined with proteins to form the two kinds of ribosomal subunits. (The proteins are manufactured on ribosomes in the cytoplasm and imported into the nucleus.) Most of these subunits leave the nucleus through the nuclear pores and enter the cytoplasm, where they join to form functional ribosomes. (a) 1 DNA double helix (2-nm diameter) Chapter Cells: The Living Units 9 Histones 2 Chromatin ( beads on a string ) structure with nucleosomes Linker DNA Nucleosome (10-nm diameter; eight histone proteins wrapped by two winds of the DNA double helix) Chromatin Seen through a light microscope, chromatin (kro mah-tin) appears as a fine, unevenly stained network, but special techniques reveal it as a system of bumpy threads weaving their way through the nucleoplasm. Chromatin is composed of about 0% DNA, which is traditionally called our genetic material, about 60% globular histone proteins (his tōn), and about 10% RNA chains, newly formed or forming. The fundamental units of chromatin are nucleosomes (nu kle-o-sōmz; nuclear bodies ), which consist of flattened disc-shaped cores or clusters of eight histone proteins connected like beads on a string by a DNA molecule. The DNA winds (like a ribbon of Velcro) twice around each nucleosome and continues on to the next cluster via linker DNA segments (Figure.0, top). Histones provide a physical means for packing the very long DNA molecules in a compact, orderly way, but they also play an important role in gene regulation. In a nondividing cell, for example, the presence of methyl groups on histone proteins shuts down the nearby DNA, and attachment of a phosphate group to a particular histone protein may indicate that the cell is about to commit suicide. On the other hand, addition of acetyl groups to histone exposes different DNA segments, or genes, so that they can dictate the specifications for protein synthesis or for various small RNA species. Such active chromatin segments, referred to as extended chromatin, are not usually visible under the light microscope. The generally inactive condensed chromatin segments are darker staining and so are more easily detected. Understandably, the most active body cells have much larger amounts of extended chromatin. Interestingly, particular chromatin strands Tight helical fiber (0-nm diameter) 5 Chromatid (700-nm diameter) (b) 4 Looped domain structure (00-nm diameter) Metaphase chromosome (at midpoint of cell division) Figure.0 Chromatin and chromosome structure. (a) Electron micrograph of chromatin fiber (125,000 ). (b) DNA packing in a chromosome. The levels of increasing structural complexity (coiling) from the DNA helix to the metaphase chromosome are indicated in order from the smallest ( 1 DNA double helix) to the largest and most complex ( 5 chromosome). occupy discrete regions in the nucleus called chromosome territories. Depending on the specific genes contained, and the cell and tissue type, the chromosome territory patterns change during development. At the simplest level, active and inactive genetic regions can be separated from each other or genetic expression can be enhanced or repressed by such chromatin separation. When a cell is preparing to divide, the chromatin threads coil and condense enormously to form short, barlike bodies called

34 94 UNIT 1 Organization of the Body TABLE. Parts of the Cell: Structure and Function CELL PART STRUCTURE FUNCTIONS Plasma Membrane (Figure.) Membrane made of a double layer of lipids (phospholipids, cholesterol, and so on) within which proteins are embedded. Proteins may extend entirely through the lipid bilayer or protrude on only one face. Externally facing proteins and some lipids have attached sugar groups. Serves as an external cell barrier, and acts in transport of substances into or out of the cell. Maintains a resting potential that is essential for functioning of excitable cells. Externally facing proteins act as receptors (for hormones, neurotransmitters, and so on) and in cell-to-cell recognition. Cytoplasm Cellular region between the nuclear and plasma membranes. Consists of fluid cytosol containing dissolved solutes, organelles (the metabolic machinery of the cytoplasm), and inclusions (stored nutrients, secretory products, pigment granules). Cytoplasmic Organelles Mitochondria (Figure.17) Ribosomes (Figures.18,.7.9) Rough endoplasmic reticulum (Figures.18,.9) Smooth endoplasmic reticulum (Figure.18) Golgi apparatus (Figures.19,.20) Lysosomes (Figure.21) Rodlike, double-membrane structures; inner membrane folded into projections called cristae. Dense particles consisting of two subunits, each composed of ribosomal RNA and protein. Free or attached to rough endoplasmic reticulum. Membrane system enclosing a cavity, the cisterna, and coiling through the cytoplasm. Externally studded with ribosomes. Membranous system of sacs and tubules; free of ribosomes. A stack of smooth membrane sacs and associated vesicles close to the nucleus. Membranous sacs containing acid hydrolases. Site of ATP synthesis; powerhouse of the cell. The sites of protein synthesis. Sugar groups are attached to proteins within the cisternae. Proteins are bound in vesicles for transport to the Golgi apparatus and other sites. External face synthesizes phospholipids. Site of lipid and steroid (cholesterol) synthesis, lipid metabolism, and drug detoxification. Packages, modifies, and segregates proteins for secretion from the cell, inclusion in lysosomes, and incorporation into the plasma membrane. Sites of intracellular digestion. Peroxisomes (Figure.2) Membranous sacs of oxidase enzymes. The enzymes detoxify a number of toxic substances. The most important enzyme, catalase, breaks down hydrogen peroxide. Microtubules (Figures.2.25) Microfilaments (Figures.2,.24) Cylindrical structures made of tubulin proteins. Fine filaments composed of the protein actin. Support the cell and give it shape. Involved in intracellular and cellular movements. Form centrioles and cilia and flagella, if present. Involved in muscle contraction and other types of intracellular movement, help form the cell s cytoskeleton. Intermediate filaments (Figure.2) Protein fibers; composition varies. The stable cytoskeletal elements; resist mechanical forces acting on the cell. Centrioles (Figure.25) Paired cylindrical bodies, each composed of nine triplets of microtubules. Organize a microtubule network during mitosis to form the spindle and asters. Form the bases of cilia and flagella. Inclusions Varied; includes stored nutrients such as lipid droplets and glycogen granules, protein crystals, pigment granules. Storage for nutrients, wastes, and cell products.

35 Chapter Cells: The Living Units 95 TABLE. (continued) CELL PART STRUCTURE FUNCTIONS Cellular Extensions Cilia (Figure.26,.27) Short cell-surface projections; each cilium composed of nine pairs of microtubules surrounding a central pair. Coordinated movement creates a unidirectional current that propels substances across cell surfaces. Flagella Like a cilium, but longer; only example in humans is the sperm tail. Propel the cell. Microvilli (Figure.28) Tubular extensions of the plasma membrane; contain a bundle of actin filaments. Increase surface area for absorption. Nucleus (Figure.2,.29) Largest organelle. Surrounded by the nuclear envelope; contains fluid nucleoplasm, nucleoli, and chromatin. Control center of the cell; responsible for transmitting genetic information and providing the instructions for protein synthesis. Nuclear envelope (Figure.29) Double-membrane structure pierced by pores. Outer membrane continuous with the endoplasmic reticulum. Separates the nucleoplasm from the cytoplasm and regulates passage of substances to and from the nucleus. Nucleoli (Figure.29) Dense spherical (non-membrane-bounded) bodies, composed of ribosomal RNA and proteins. Site of ribosome subunit manufacture. Chromatin (Figure.0) Granular, threadlike material composed of DNA and histone proteins. DNA constitutes the genes. chromosomes ( colored bodies ) (Figure.0, bottom). Chromosome compactness prevents the delicate chromatin strands from tangling and breaking during the movements that occur during cell division. We describe the functions of DNA and the events of cell division in the next section. Table. summarizes the parts of the cell. CHECK YOUR UNDERSTANDING 2. If a cell ejects or loses its nucleus, what is its fate and why? 24. What is the role of nucleoli? 25. What is the importance of the histone proteins present in the nucleus? For answers, see Appendix G. Cell Growth and Reproduction The Cell Life Cycle List the phases of the cell life cycle and describe the key events of each phase. Describe the process of DNA replication. The cell life cycle is the series of changes a cell goes through from the time it is formed until it reproduces. The outer ring of Figure.1 shows the two major periods of the cell cycle: interphase (in green), in which the cell grows and carries on its usual activities, and cell division or the mitotic phase (in yellow), during which it divides into two cells. Interphase Interphase is the period from cell formation to cell division. Early cytologists, unaware of the constant molecular activity in cells and impressed by the obvious movements of cell division, called interphase the resting phase of the cell cycle. (The term interphase reflects this idea of a stage between cell divisions.) However, this image is grossly misleading because during interphase a cell is carrying out all its routine activities and is resting only from dividing. Perhaps a more accurate name for this phase would be metabolic phase or growth phase. Subphases In addition to carrying on its life-sustaining reactions, an interphase cell prepares for the next cell division. Interphase is divided into G 1, S, and G 2 subphases (the Gs stand for gaps before and after the S phase, and S is for synthetic). In all three subphases, the cell grows by producing proteins and organelles, but chromatin is reproduced only during the S subphase. During G 1 (gap 1), the cell is metabolically active, synthesizing proteins rapidly and growing vigorously (Figure.1, light green area). This is the most variable phase in terms of length. In cells that divide rapidly, G 1 typically lasts several minutes to hours, but in those that divide slowly, it may last for days or even

36 96 UNIT 1 Organization of the Body G 1 checkpoint (restriction point) G 1 Growth Cytokinesis Interphase S Growth and DNA synthesis G 2 Growth and final preparations for M division Telophase Mitosis years. Cells that permanently cease dividing are said to be in the G 0 phase. For most of G 1, virtually no activities directly related to cell division occur. However, as G 1 ends, the centrioles start to replicate in preparation for cell division. During the S phase, DNA is replicated, ensuring that the two future cells being created will receive identical copies of the genetic material (Figure.1, blue area). New histones are made and assembled into chromatin. One thing is sure: Without a proper S phase, there can be no correct mitotic phase. (We will describe DNA replication next.) The final phase of interphase, called G 2, is brief (Figure.1, dark green area). Enzymes and other proteins needed for division are synthesized and moved to their proper sites. By the end of G 2, centriole replication (begun in G 1 ) is complete. The cell is now ready to divide. Throughout S and G 2, the cell continues to grow and carries on with business as usual. DNA Replication Before a cell can divide, its DNA must be replicated exactly, so that identical copies of the cell s genes can be passed on to each of its offspring. During the S phase, replication begins simultaneously on several chromatin threads and continues until all the DNA has been replicated. Anaphase Metaphase Prophase Mitotic phase (M) G 2 checkpoint Figure.1 The cell cycle. During G 1, cells grow rapidly and carry out their routine functions. The S phase is the period of DNA synthesis. In G 2, materials needed for cell division are synthesized and growth continues. During the M phase (cell division), mitosis and cytokinesis occur, producing two daughter cells. Important checkpoints at which mitosis may be prevented from occurring are found throughout interphase; two are shown on the diagram. Replication is still being studied but appears to involve the following events: 1. The DNA helices begin unwinding from the histones of the nucleosomes. 2. In an ATP-requiring process, a helicase enzyme untwists the double helix and gradually separates the DNA molecule into two complementary nucleotide chains, exposing the nitrogenous bases. The Y-shaped site of separation, where active DNA replication will soon begin, is called the replication fork (Figure.2).. Each nucleotide strand then serves as a template,or set of instructions, for building a new complementary nucleotide strand from free DNA precursors dissolved in the nucleoplasm. 4. At sites where DNA synthesis is to occur, the needed machinery gradually accumulates until several different proteins (mostly enzymes) are present in a large complex called a replisome. Primase enzymes, which are part of the replisome, catalyze the formation of short (about ten bases long) RNA primers, which actually initiate DNA synthesis. 5. Once the primer is in place, the enzyme DNA polymerase comes into the picture. Continuing from the primer, it positions complementary nucleotides along the template strand and then covalently links them together. So, if the DNA polymerase encounters the sequence of bases GCT on the template strand, it assembles the bases CGA to bind to it. DNA polymerase works only in one direction. Consequently, one strand, the leading strand, is synthesized continuously (once primed by the RNA primer) following the movement of the replication fork. The other strand, called the lagging strand, is constructed in segments in the opposite direction and requires that a primer initiate replication of each segment. These primers are eventually replaced with DNA nucleotides by DNA polymerases. 6. The short segments of DNA are then spliced together by DNA ligase. The end result is that two DNA molecules are formed from the original DNA helix and are identical to it. Because each new molecule consists of one old and one new nucleotide strand, this mechanism of DNA replication is called semiconservative replication (Figure.2). Replication also involves the generation of two new telomeres (tel end; mer piece), snugly fitting nucleoprotein caps that prevent degradation of the ends of the chromatin strands. As soon as replication ends, histones (synthesized in the cytoplasm and imported into the nucleus) associate with the DNA, completing the formation of two new chromatin strands. The chromatin strands, united by a buttonlike centromere (a stretch of repetitive DNA), remain attached, held together by the centromere and a protein complex called cohesin, until the cell enters the anaphase stage of mitotic cell division (see p. 99). They are then distributed to the daughter cells as described next, ensuring that each cell has identical genetic information. The progression from DNA synthesis into the events of cell division presumes that the newly synthesized DNA is not damaged or broken in any way. If damage occurs, progress through

37 Chapter Cells: The Living Units 97 Chromosome Free nucleotides DNA polymerase Old strand acts as a template for synthesis of new strand Leading strand Two new strands (leading and lagging) synthesized in opposite directions Old DNA Helicase unwinds the double helix and exposes the bases Replication fork Lagging strand Adenine Thymine Cytosine Guanine DNA polymerase Old (template) strand Figure.2 Replication of DNA. The DNA helix uncoils, and the hydrogen bonds between its base pairs are broken. Then, each nucleotide strand of the DNA acts as a template for constructing a complementary strand, as illustrated on the right-hand side of the diagram. DNA polymerases work in one direction only, so the two new strands (leading and lagging) are synthesized in opposite directions. (The DNA ligase enzymes that join the DNA fragments on the lagging strand are not illustrated.) Each DNA molecule formed consists of one old (template) strand and one newly assembled strand and constitutes a chromatid of a chromosome. the cell cycle is arrested until the DNA repair mechanism has fixed the problem. Cell Division Cell division is essential for body growth and tissue repair. Cells that continually wear away, such as cells of the skin and intestinal lining, reproduce themselves almost continuously. Others, such as liver cells, divide more slowly (to maintain the particular size of the organ they compose) but retain the ability to reproduce quickly if the organ is damaged. Most cells of nervous tissue, skeletal muscle, and heart muscle lose their ability to divide when they are fully mature, and repairs are made with scar tissue (a fibrous type of connective tissue). Events of Cell Division In most body cells, cell division, which is called the M (mitotic) phase of the cell life cycle, involves two distinct events: mitosis (mi-to sis; mit thread; osis process), or division of the nucleus, and cytokinesis (si-to-kĭne sis; kines movement), or division of the cytoplasm (see Figure.1, yellow area). A somewhat different process of nuclear division called meiosis (mi-o sis) produces sex cells (ova and sperm) with only half the number of genes found in other body cells. We discuss the details of meiosis in Chapter 27. Here we concentrate on mitotic cell division. Mitosis Mitosis is the series of events that parcel out the replicated DNA of the mother cell to two daughter cells. It is described in terms of four phases: prophase, metaphase, anaphase, and telophase, but it is actually a continuous process, with one phase merging smoothly into the next. Its duration varies according to cell type, but in human cells it typically lasts about an hour or less from start to finish. Focus on Mitosis (Figure.), pp , describes the phases of mitosis in detail. Cytokinesis Cytokinesis, or the division of the cytoplasm, begins during late anaphase and is completed after mitosis ends. The plasma membrane over the center of the cell (the spindle equator) is drawn inward to form a cleavage furrow by the activity of a contractile ring (Figure.) made of actin filaments. The furrow deepens until the cytoplasmic mass is pinched into two parts, so that at the end of cytokinesis there are two daughter cells. Each is smaller and has less cytoplasm than the mother

38 Figure. FOCUS Mitosis Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it produces two identical daughter cells. Interphase Early Prophase Late Prophase Centrosomes (each has 2 centrioles) Plasma membrane Early mitotic spindle Aster Spindle pole Polar microtubule Fragments of nuclear envelope Nucleolus Nuclear envelope Chromatin Chromosome consisting of two sister chromatids Centromere Kinetochore Kinetochore microtubule Interphase Interphase is the period of a cell s life when it carries out its normal metabolic activities and grows. During interphase, the DNA-containing material is in the form of chromatin. The nuclear envelope and one or more nucleoli are intact and visible. There are three distinct periods of this phase: G 1 : The centrioles begin replicating. S: DNA is replicated. G 2 : Final preparations for mitosis are completed and centrioles finish replicating. The light micrographs show dividing lung cells from a newt. The chromosomes appear blue and the microtubules green. (The red fibers are intermediate filaments.) The schematic drawings show details not visible in the micrographs. For simplicity, only four chromosomes are drawn. Early Prophase The chromatin condenses, forming barlike chromosomes that are visible with a light microscope. Each duplicated chromosome appears as two identical threads, now called sister chromatids, held together at a small, constricted region called a centromere. (After the chromatids separate, each is considered a new chromosome.) As the chromosomes appear, the nucleoli disappear, and the two centrosomes separate from one another. The centrosomes act as focal points for growth of a microtubule assembly called the mitotic spindle. As these microtubules lengthen, they propel the centrosomes toward opposite ends (poles) of the cell. Microtubule arrays called asters ( stars ) are seen extending from the matrix around the centrosomes. Late Prophase While the centrosomes are still moving apart, the nuclear envelope fragments, allowing the spindle to interact with the chromosomes. Some of the growing spindle microtubules attach to kinetochores (ki-ne -to-korz), special protein structures at each chromosome s centromere. Such microtubules are called kinetochore microtubules. The remaining spindle microtubules (not attached to any chromosomes) are called polar microtubules. The microtubules slide past each other, forcing the poles apart. The kinetochore microtubules pull on each chromosome from both poles in a kind of tug-of-war that ultimately draws the chromosomes to the exact center or equator of the cell. 98

39 Metaphase Anaphase Telophase and Cytokinesis Spindle Nuclear envelope forming Nucleolus forming Contractile ring at cleavage furrow Metaphase plate Daughter chromosomes Metaphase Metaphase is the second phase of mitosis. The two centrosomes are at opposite poles of the cell. The chromosomes cluster at the middle of the cell, with their centromeres precisely aligned at the equator of the spindle. This imaginary plane midway between the poles is called the metaphase plate. Anaphase Anaphase is the third and shortest phase of mitosis. Anaphase begins abruptly as the centromeres of the chromosomes split simultaneously. Each chromatid now becomes a chromosome in its own right. The kinetochore microtubules, moved along by motor proteins in the kinetochores, gradually pull each chromosome toward the pole it faces. At the same time, the polar microtubules slide past each other, lengthen, and push the two poles of the cell apart. Anaphase is easy to recognize because the moving chromosomes look V shaped. The centromeres lead the way, and the chromosomal arms dangle behind them. This process of moving and separating the chromosomes is helped by the fact that the chromosomes are short, compact bodies. Diffuse threads of chromatin would tangle, trail, and break, resulting in imprecise parceling out to the daughter cells. Telophase Telophase begins as soon as chromosomal movement stops. This final phase is like prophase in reverse. The identical sets of chromosomes at the opposite poles of the cell uncoil and resume their threadlike chromatin form. A new nuclear envelope forms around each chromatin mass, nucleoli reappear within the nuclei, and the spindle breaks down and disappears. Mitosis is now ended. The cell, for just a brief period, is binucleate (has two nuclei) and each new nucleus is identical to the original mother nucleus. Cytokinesis As a rule, as mitosis draws to a close, cytokinesis completes the division of the cell into two identical daughter cells. Cytokinesis occurs as a contractile ring of actin microfilaments forms the cleavage furrow and pinches the cell apart. It begins during late anaphase and continues through and beyond telophase. 99

40 100 UNIT 1 Organization of the Body cell, but is genetically identical to it. The daughter cells then enter the interphase portion of the life cycle until it is their turn to divide. Control of Cell Division The signals that prod cells to divide are incompletely understood, but we know that the ratio of cell surface area to cell volume is important. The amount of nutrients a growing cell requires is directly related to its volume. Volume increases with the cube of cell radius, whereas surface area increases more slowly with the square of the radius. For example, a 64-fold (4 ) increase in cell volume is accompanied by only a 16-fold (4 2 ) increase in surface area. Consequently, the surface area of the plasma membrane becomes inadequate for nutrient and waste exchange when a cell reaches a certain critical size. Cell division solves this problem because the smaller daughter cells have a favorable ratio of surface area to volume. These surface-volume relationships help explain why most cells are microscopic in size. Two other factors that influence when cells divide are chemical signals (growth factors, hormones, and others) released by other cells and the availability of space. Normal cells stop proliferating when they begin touching, a phenomenon called contact inhibition. The cell life cycle is controlled by a system that has been compared to an automatic washer s timer control. Like that timer, the control system for the cell cycle is driven by a built-in clock. However, just as the washer s cycle is subject to adjustments (by regulating the flow from the faucet, say, or by an internal water-level sensor), the cell cycle is regulated by both internal and external factors. Two groups of proteins are crucial to the ability of a cell to accomplish the S phase and enter mitosis. They are cyclins (regulatory proteins whose levels rise and fall during each life cycle) and Cdks (cyclin-dependent kinases), which are present in a constant concentration in the cell and are activated by binding to particular cyclins. In response to specific signals, a new batch of cyclins accumulates during each interphase. Subsequent joining of specific Cdk and cyclin proteins initiates enzymatic cascades that phosphorylate histones and other proteins needed for the various stages of cell division. At the end of mitosis, the cyclins are abruptly destroyed by enzymes. A number of switches and crucial checkpoints for cell division occur throughout interphase. These built-in stop signals halt the cell cycle until overridden by internal or external go-ahead signals. In many cells, a G 1 checkpoint, called the restriction point, seems to be most important (see Figure.1). If the cell is diverted from dividing at this checkpoint, it enters the nondividing state (G 0 ). Another important checkpoint, and the first to be understood, occurs late in G 2,when a threshold amount of a protein complex called MPF (M-phase promoting factor) is required to give the okay signal to pass the G 2 checkpoint and enter the M phase. Later in M phase, MPF is inactivated. Besides these go signals, there are a number of so-called repressor genes that inhibit cell division. One example is the p5 gene that initiates a series of enzymatic events that produce growth-inhibiting factors. Roughly half of all cancers have abnormal p5 genes. These cancers are not inhibited by contact with other cells and divide wildly, making them dangerous to their host. CHECK YOUR UNDERSTANDING 26. If one of the DNA strands being replicated reads CGAATG, what will be the base sequence of the corresponding DNA strand? 27. During what phase of the cell cycle is DNA synthesized? 28. What are three events occurring in prophase that are undone in telophase? For answers, see Appendix G. Protein Synthesis Define gene and genetic code and explain the function of genes. Name the two phases of protein synthesis and describe the roles of DNA, mrna, trna, and rrna in each phase. Contrast triplets, codons, and anticodons. In addition to directing its own replication, DNA serves as the master blueprint for protein synthesis. Although cells also make lipids and carbohydrates, DNA does not dictate their structure. Historically, DNA is said to specify only the structure of protein molecules, including the enzymes that catalyze the synthesis of all classes of biological molecules. Most of the metabolic machinery of the cell is concerned in some way with protein synthesis. This is not surprising, seeing as structural proteins constitute most of the dry cell material, and functional proteins direct almost all cellular activities. Essentially, cells are miniature protein factories that synthesize the huge variety of proteins that determine the chemical and physical nature of cells and therefore of the whole body. Proteins, as you will recall from Chapter 2, are composed of polypeptide chains, which in turn are made up of amino acids. For purposes of this discussion, we can define a gene as a segment of a DNA molecule that carries instructions for creating one polypeptide chain. (Note, however, that some genes specify the structure of certain varieties of RNA as their final product.) The four nucleotide bases (A, G, T, and C) are the letters used in the genetic dictionary, and the information of DNA is found in the sequence of these bases. Each sequence of three bases, called a triplet, can be thought of as a word that specifies a particular amino acid. For example, the triplet AAA calls for the amino acid phenylalanine, and CCT calls for glycine. The sequence of triplets in each gene forms a sentence that tells exactly how a particular polypeptide is to be made: It specifies the number, kinds, and order of amino acids needed to build a particular polypetide. Variations in the arrangement of A, T, C, and G allow our cells to make all the different kinds of proteins needed. Even an unusually small gene has an estimated 210 base pairs in sequence. The ratio between DNA bases in the gene and amino acids in the polypeptide is :1 (because each triplet stands for

41 one amino acid), so we would expect the polypeptide specified by such a gene to contain 70 amino acids. Most genes of higher organisms contain exons, which are amino acid specifying informational sequences. These exons are often separated by introns, which are noncoding, often repetitive, segments that range from 60 to 100,000 nucleotides. Once considered a type of junk DNA, intron DNA is now believed to represent a genome scrapyard that provides a reservoir of ready-to-use DNA segments for genome evolution, as well as a rich source of a large variety of small RNA molecules. The rest of the DNA (the great majority of it) is essentially dark matter whose function is a mystery. It is in these regions that pseudogenes (false genes) are found. Pseudogenes look like real genes but have deficits that render them apparently functionless. The Role of RNA By itself, DNA is like a CD recording: The information it contains cannot be used without a decoding mechanism (a CD player). Furthermore, most polypeptides are manufactured at ribosomes in the cytoplasm, but in interphase cells, DNA never leaves the nucleus. So, DNA requires not only a decoder, but a messenger as well. The decoding and messenger functions are carried out by RNA, the second type of nucleic acid. As you learned in Chapter 2, RNA differs from DNA in being single stranded and in having the sugar ribose instead of deoxyribose and the base uracil (U) in place of thymine (T). Three forms of RNA typically act together to carry out DNA s instructions for polypeptide synthesis: 1. Messenger RNA (mrna), relatively long nucleotide strands resembling half-dna molecules (one of the two strands of a DNA molecule coding for protein structure) 2. Ribosomal RNA (rrna), part of the ribosomes. Transfer RNA (trna), small, roughly L-shaped molecules All types of RNA are formed on the DNA in the nucleus in much the same way as DNA replicates itself: The DNA helix separates and one of its strands serves as a template for synthesizing a complementary RNA strand. Once formed, the RNA molecule is released from the DNA template and migrates into the cytoplasm. Its job done, the DNA simply recoils into its helical, inactive form. Approximately 2% of the nuclear DNA codes for the synthesis of short-lived mrna, so named because it carries, from gene to ribosome, the message containing the instructions for building a polypeptide. (Kind of a genetic memo for protein structure.) DNA in the nucleolar organizer regions (mentioned previously) codes for the synthesis of rrna, which is long-lived and stable, as is trna coded by other DNA sequences. Because rrna and trna do not transport codes for synthesizing other molecules, they are the final products of the genes that code for them. Ribosomal RNA and trna act together to translate the message carried by mrna. Essentially, polypeptide synthesis involves two major steps: (1) transcription, in which DNA s information is encoded Chapter Cells: The Living Units 101 Transcription RNA Processing Translation mrna Polypeptide DNA Pre-mRNA Ribosome Nuclear envelope Nuclear pores Figure.4 Simplified scheme of information flow from the DNA gene to mrna to protein structure during transcription and translation. (Note that mrna is first synthesized as pre-mrna, which is processed by enzymes before leaving the nucleus.) in mrna, and (2) translation, in which the information carried by mrna is decoded and used to assemble polypeptides. Figure.4 shows an overview of the information flow in these two major steps. The figure also indicates the RNA processing that removes introns from mrna before this molecule leaves the nucleus and moves into the cytoplasm. Transcription A transcriptionist converts a message from a recording or shorthand notes into a word-processed or electronic copy. In other words, information is transformed, or transferred, from one form or format to another. In cells, transcription involves the transfer of information from a DNA s base sequence to the complementary base sequence of an mrna molecule. The form is different, but the same information is being conveyed. Once the mrna molecule is made, it detaches and leaves the nucleus via a nuclear pore. Essentially, three basic phases are involved in transcription: (1) initiation, (2) elongation, and () termination. In addition to looking at the synthesis of mrna, we will also examine the editing and further processing needed to clean up the mrna transcript. How does transcription get started? Transcription cannot begin until gene-activating chemicals called transcription factors stimulate loosening of the histones at the site-to-be of gene

42 102 UNIT 1 Organization of the Body RNA polymerase Figure.5 Overview of stages of transcription. DNA Coding strand Promoter region Template strand Termination signal 1 Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mrna synthesis at the start point on the template strand. mrna Template strand 2 Elongation: As the RNA polymerase moves along the template strand, elongating the mrna transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it. Rewinding of DNA Coding strand of DNA RNA nucleotides Direction of transcription Unwinding of DNA mrna transcript mrna DNA-RNA hybrid region Template strand Termination: mrna synthesis ends when the termination signal is reached. RNA polymerase and the completed mrna transcript are released. RNA polymerase The DNA-RNA hybrid: At any given moment, base pairs of DNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid. Completed mrna transcript RNA polymerase transcription and then bind to the promoter. The promoter is a special DNA sequence that contains the start point (beginning of the structural gene to be transcribed). It specifies where mrna synthesis starts and which DNA strand is going to serve as the template strand (Figure.5, top). The uncoiled DNA strand not used as a template is called the coding strand because it has the same (coded) sequence as the mrna to be built (except for the use of U in mrna in place of T in DNA). The transcription factors also help position RNA polymerase, the enzyme that oversees the synthesis of mrna, correctly at the promoter. Once these preparations are made, RNA polymerase can initiate transcription. Figure.5 illustrates the following steps in transcription: 1 Initiation. Once bound with the help of the transcription factors, RNA polymerase pulls apart the strands of the 2 DNA double helix so that transcription can begin at the start point in the promoter. Elongation. Using incoming RNA nucleotides as substrates, the RNA polymerase aligns them with complementary DNA bases on the template strand and then links them together. As RNA polymerase elongates the mrna strand one base at a time, it unwinds the DNA helix in front of it, and rewinds the helix behind it. At any given moment, 16 to 18 base pairs of DNA are unwound and the most recently made mrna is still hydrogen-bonded (H-bonded) to the template DNA. This small region called the DNA-RNA hybrid is up to 12 base pairs long. Termination. When the polymerase reaches a special base sequence called a termination signal, transcription ends and the newly formed mrna pulls off the DNA template.

43 Processing of mrna Although it would appear that translation can begin as soon as the mrna is made, that is not the case. The process is a bit more complex. Recall that mammalian DNA like ours has coding regions (exons) separated by non-proteincoding regions (introns). Because the DNA is transcribed sequentially, the mrna initially made, called pre-mrna or primary transcript, is littered with intron junk or nonsense. Before the newly formed RNA can be used as a messenger, it must be processed, or edited that is, sections corresponding to introns must be removed. This job is done by spliceosomes. These large RNA-protein complexes snip out the introns and splice together the remaining exon-coded sections in the order in which they occurred in the DNA, producing the functional mrna that directs translation at the ribosome. Although many introns naturally degrade, some contain active segments (such as micrornas) that can function to control, interfere with, or silence other genes. Additionally, before the edited mrna can function in protein synthesis, a number of specific RNA-binding proteins, called mrna complex proteins, must become associated with it. These mrna complex proteins guide its export from the nucleus, determine its localization, translation, and stability, and check it for premature termination codons. Translation A translator takes a message in one language and restates it in another. In the translation step of protein synthesis, the language of nucleic acids (base sequence) is translated into the language of proteins (amino acid sequence). The rules by which the base sequence of a gene is translated into an amino acid sequence are called the genetic code. For each triplet, or three-base sequence on DNA, the corresponding three-base sequence on mrna is called a codon. Since there are four kinds of RNA (or DNA) nucleotides, there are 4, or 64, possible codons. Three of these 64 codons are stop signs that call for termination of polypeptide synthesis. All the rest code for amino acids. Because there are only about 20 amino acids, some are specified by more than one codon. This redundancy in the genetic code helps protect against problems due to transcription (and translation) errors. The genetic code and a complete codon list are provided in Figure.6. The process of translation occurs in the cytoplasm and involves the mrnas, trnas, and rrnas mentioned above, and occurs in the sequence of events described next and illustrated in Figure.7. When it reaches the cytoplasm after processing in the nucleus, the mrna molecule carrying instructions for a particular protein binds to a small ribosomal subunit (Figure.7, 1 ). Then trna comes into the picture, its job being to transfer amino acids, dissolved in the cytosol, to the ribosome. Shaped like a handheld drill, trna is well suited to its dual function of binding to both an amino acid and an mrna codon. The amino acid is bound to one end of trna, at a region called the stem. At the other end, the head, is its anticodon (an ti-ko don), a three-base sequence complementary to the mrna codon calling for the amino acid carried by that partic- FIRST BASE U C A G UUU UUC UUA UUG CUU CUC CUA CUG AUU AUC AUA AUG GUU GUC GUA GUG Chapter Cells: The Living Units 10 Phe Leu Leu Ile Met or Start Val UCU UCC UCA UCG CCU CCC CCA CCG ACU ACC ACA ACG GCU GCC GCA GCG SECOND BASE U C A G Ser Pro Thr Ala UAU UAC CAU CAC CAA CAG AAU AAC AAA AAG GAU GAC GAA GAG Tyr His Gln Asn Lys Asp Glu UGU UGC UAA Stop UGA Stop UAG Stop UGG CGU CGC CGA CGG AGU AGC AGA AGG GGU GGC GGA GGG Cys Figure.6 The genetic code. The three bases in an mrna codon are designated as the first, second, and third. Each set of three specifies a particular amino acid, represented here by an abbreviation (see list below). The codon AUG (which specifies the amino acid methionine) is the usual start signal for protein synthesis. The word stop indicates the codons that serve as signals to terminate protein synthesis. Abb.* Amino acid Abb.* Amino acid Ala alanine Leu leucine Arg arginine Lys lysine Asn asparagine Met methionine Asp aspartic acid Phe phenylalanine Cys cysteine Pro proline Glu glutamic acid Ser serine Gln glutamine Thr threonine Gly glycine Trp tryptophan His histidine Tyr tyrosine Ile isoleucine Val valine *Abbreviation for the amino acid ular trna. Because anticodons form hydrogen bonds with complementary codons, trna is the link between the language of nucleic acids and the language of proteins. For example, if the mrna codon is AUA, which specifies isoleucine, the trnas carrying isoleucine will have the anticodon UAU, which can bind to the AUA codons. There are approximately 45 different types of trna, each capable of binding with a specific amino acid. The attachment Trp Arg Ser Arg Gly U C A G U C A G U C A G U C A G THIRD BASE

44 104 UNIT 1 Organization of the Body Nucleus RNA polymerase Energized by ATP, the correct amino acid is attached to each species of trna by aminoacyl-trna synthetase enzyme. mrna Template strand of DNA Amino acid Leu 1 After mrna synthesis in the nucleus, mrna leaves the nucleus and attaches to a ribosome. Nuclear membrane Nuclear pore trna G A A 2 Translation begins as incoming aminoacyl-trna recognizes the complementary codon calling for it at the A site on the ribosome. It hydrogen-bonds to the codon via its anticodon. Released mrna Aminoacyl-tRNA synthetase Leu As the ribosome moves along the mrna, and each codon is read in sequence, a new amino acid is added to the growing protein chain and the trna in the A site is translocated to the P site. Ile Pro G A A trna head bearing anticodon 4 Once its amino acid is released from the P site, trna is ratcheted to the E site and then released to reenter the cytoplasmic pool, ready to be recharged with a new amino acid. The polypeptide is released when the stop codon is read. U A U A E site U A P site G G C C C G A site C U U Large ribosomal subunit Codon 15 Codon 16 Codon 17 Small ribosomal subunit Portion of mrna already translated Direction of ribosome advance Figure.7 The basic steps of translation. The diagram presumes that initiation was accomplished and the 17th amino acid as dictated by the mrna codons is being brought to the ribosome. process is controlled by an aminoacyl-trna synthetase enzyme and is activated by ATP (see the top right side of Figure.7). Once its amino acid is loaded, the trna (now called an aminoacyl-trna because of its amino acid cargo) migrates to the ribosome, where it maneuvers the amino acid into the proper position, as specified by the mrna codons (Figure.7, 2 ). The ribosome is more than just a passive attachment site for mrna and trna. Like a vise, the ribosome holds the trna and mrna close together to coordinate the coupling of codons and anticodons, and positions the next (incoming) amino acid for addition to the growing polypeptide chain. To do its job, the ribosome has a binding site for mrna and three binding sites

45 for trna: an A (aminoacyl) site for an incoming aminoacyltrna, a P (peptidyl) site for the trna holding the growing polypeptide chain, and an E (exit) site for an outgoing trna (Figure.7). How does translation get started? When a special methioninecharged initiator trna binds to the middle (P) site on the small ribosomal subunit, the translation process officially begins. The newly formed mrna has an initial base sequence, called a leader sequence, that allows it to attach to its binding site on the small ribosomal subunit. With the initiator trna still in tow, the small ribosomal subunit scans along the mrna until it encounters the start codon (the first AUG triplet it meets). When the initiator trna s UAC anticodon recognizes and binds to the start codon, a large ribosomal subunit unites with the small one, forming a functional ribosome. With the mrna firmly positioned in the groove between the two ribosomal subunits, translation begins in earnest. This initiation process requires the help of a number of initiation factors and is energized by GTP. Now the ribosome slides the mrna strand along, bringing the next codon into position to be read by an aminoacyl-trna coming into the A site (Figure.7, 2 ). It is at this point that the ribosome does a little proofreading to make sure of the codonanticodon match. That accomplished, an enzymatic component in the large ribosomal particle peptide-bonds the amino acid of the initiator trna in the P site to that of the trna at the A site. The ribosome then translocates the trna that is now carrying two amino acids to the P site (as in Figure.7, ). At the same time, it ratchets the initiator trna to the E site, from which it leaves the ribosome (as in Figure.7, 4 ). This orderly musical chairs process continues: the peptidyltrnas transferring their polypeptide cargo to the aminoacyltrnas, and then the P-site-to-E-site and A-site-to-P-site movements of the trnas (see again Figure.7, and 4 ). As the ribosome chugs along the mrna track and the mrna is progressively read, its initial portion passes through the ribosome and may ultimately become attached successively to several other ribosomes, all reading the same message simultaneously and sequentially. Such a multiple ribosome mrna complex is called a polyribosome, and it provides an efficient system for producing many copies of the same protein (Figure.8). The mrna strand continues to be read sequentially until its last codon, the stop codon (one of UGA, UAA, or UAG) enters the ribosomal groove. The stop codon is the period at the end of the mrna sentence that tells the ribosome that translation of that mrna is finished. The completed polypeptide chain is then released from the ribosome, and the ribosome separates into its two subunits (Figure.8a). The released protein folds into a complex -D structure and floats off, ready to work. When the message of the mrna that directed its formation becomes outdated, it is degraded in structures called processing (P) bodies. As noted earlier in the chapter, ribosomes attach to and detach from the rough ER. When a short leader peptide called an ER signal sequence is present in a protein being synthesized, the associated ribosome attaches to the membrane of the rough ER. This signal sequence, with its attached cargo of a ribosome and mrna, is guided to appropriate receptor sites on the ER membrane by a signal-recognition particle (SRP), a molecular Incoming ribosomal subunits chaperone that cycles between the ER and the cytosol. The subsequent events occurring at the ER are detailed in Figure.9. In summary, the genetic information of a cell is translated into the production of proteins via a sequence of information transfer that is completely directed by complementary base pairing. The transfer of information goes from DNA base sequence (triplets) to the complementary base sequence of mrna (codons) and then to the trna base sequence (anticodons), which is identical to the DNA sequence except for the substitution of uracil (U) for thymine (T) (Figure.40). Other Roles of DNA The story of DNA doesn t end with the production of proteins encoded by exons. Scientists are finding that other intron or junk DNA actually codes for a surprising variety of active RNA species, including the following: Start of mrna Chapter Cells: The Living Units 105 Growing polypeptides Polyribosome End of mrna Completed polypeptide (a) Each polyribosome consists of one strand of mrna being read by several ribosomes simultaneously. In this diagram, the mrna is moving to the left and the oldest functional ribosome is farthest to the right. (b) This transmission electron micrograph shows a large polyribosome (400,000 ). Ribosomes mrna Figure.8 Polyribosome arrays allow a single strand of mrna to be translated into hundreds of the same polypeptide molecules in a short time. Antisense RNAs, made on the DNA strand complementary to the template strand for mrna, can intercept and bind to the protein-coding mrna strand and prevent it from being translated into protein. MicroRNAs are small RNAs that can use RNA interference machinery to interfere with and suppress mrnas made by certain exons, thus effectively silencing them. Riboswitches are folded RNAs that code, like mrna, for a particular protein. What sets them apart from other mrnas is

46 106 UNIT 1 Organization of the Body 1 The mrna-ribosome complex is directed to the rough ER by the SRP. There the SRP binds to a receptor site. 2 Once attached to the ER, the SRP is released and the growing polypeptide snakes through the ER membrane pore into the cisterna. ER signal sequence Ribosome mrna The signal sequence is clipped off by an enzyme. As protein synthesis continues, sugar groups may be added to the protein. Signal recognition particle (SRP) Receptor site Growing polypeptide Signal sequence removed Released protein Sugar group 4 In this example, the completed protein is released from the ribosome and folds into its -D conformation, a process aided by molecular chaperones. 5 The protein is enclosed within a protein (coatomer)-coated transport vesicle. The transport vesicles make their way to the Golgi apparatus, where further processing of the proteins occurs (see Figure.19). Rough ER cisterna Cytoplasm Transport vesicle pinching off Coatomer-coated transport vesicle Figure.9 Presence of an endoplasmic reticulum (ER) signal sequence in a newly forming protein causes the signal recognition particle (SRP) to direct the mrna-ribosome complex to the rough ER. a region that acts as a switch to turn protein synthesis on or off in response to specific metabolic changes in the environment, such as changes in the concentration of vitamins, amino acids, nucleotides, or other small molecules in the cell. When it senses these changes, the riboswitch changes shape, thereby stopping or starting production of the protein it specifies. We still have much to learn about these versatile RNA species that arise from intron and junk DNA and appear to play a role in heredity. Other areas of research focus on the consequences of the triplet code being degenerate (that is, having several codons code for a single amino acid), and on the multitasking of DNA segments. For example, a sequence can code for structure of a protein and also manage to guide positioning of a nucleosome. CHECK YOUR UNDERSTANDING 29. Codons and anticodons are both three-base sequences. How do they differ? 0. How do the A, P, and E ribosomal sites differ functionally during protein synthesis? 1. What is the role of DNA in transcription? For answers, see Appendix G. Cytosolic Protein Degradation Describe the importance of ubiquitin-dependent degradation of soluble proteins. What happens to proteins that are no longer useful? To carry out their physiological roles, proteins must be in the right place at the right time and in the right amounts. Like everything else, proteins outlive their usefulness and are eventually degraded. Organelle proteins are digested by lysosomes, but lysosomal enzymes do not have access to misfolded, damaged, or unneeded soluble proteins in the cytosol that need to be disposed of. For example, these proteins might include some used only in cell division, or short-lived transcription factors. So, how does the cell prevent such proteins from accumulating while avoiding wholesale destruction of virtually all soluble proteins by the cytosolic enzymes? Doomed proteins are marked for attack (proteolysis) by attachment of proteins called ubiquitins (u-bĭ kwĭ-tinz) in an ATP-dependent reaction. The tagged proteins are then hydrolyzed to small peptides by soluble enzymes or by proteasomes, giant waste disposal complexes composed of protein-digesting enzymes and the ubiquitin is

47 Chapter Cells: The Living Units 107 DNA molecule Gene 1 Gene 2 Gene 4 DNA: DNA base sequence (triplets) of the gene codes for synthesis of a particular polypeptide chain Triplets 1 T A C G G T A G C G A T T T C C C T G C G A A A A C T mrna: Base sequence (codons) of the transcribed mrna trna: Consecutive base sequences of trna anticodons able to recognize the mrna codons calling for the amino acids they transport Polypeptide: Amino acid sequence of the polypeptide chain Codons A U G C C A U C G C U A A A G G G A C G C U U U U G A Anticodon U A C G G U A G C G A U U U C C C U G C G A A A trna Met Pro Ser Leu Lys Gly Arg Phe Start Stop; detach Figure.40 Information transfer from DNA to RNA to polypeptide. Information is transferred from the DNA of the gene to the complementary messenger RNA molecule, whose codons are then read by transfer RNA anticodons. Notice that the reading of the mrna by trna anticodons reestablishes the base (triplet) sequence of the DNA genetic code (except that T is replaced by U). recycled. Proteasome activity is critical during starvation when these complexes degrade preexisting proteins to provide amino acids for synthesis of new and needed proteins. Extracellular Materials Name and describe the composition of extracellular materials. Extracellular materials are any substances contributing to body mass that are found outside the cells. One class of extracellular materials is body fluids, mainly interstitial fluid, blood plasma, and cerebrospinal fluid. These fluids are important transport and dissolving media. Cellular secretions are also extracellular materials and include substances that aid in digestion (intestinal and gastric fluids) and some that act as lubricants (saliva, mucus, and serous fluids). By far the most abundant extracellular material is the extracellular matrix. Most body cells are in contact with a jellylike substance composed of proteins and polysaccharides. These molecules are secreted by the cells and self-assemble into an organized mesh in the extracellular space, where they serve as a universal cell glue that helps to hold body cells together. As described in Chapter 4, the extracellular matrix is particularly abundant in connective tissues in some cases so abundant that it (rather than living cells) accounts for the bulk of that tissue type. Depending on the structure to be formed, the extracellular matrix in connective tissue ranges from soft to rock-hard. CHECK YOUR UNDERSTANDING 2. What is the importance of ubiquitin in the life of a cell?. What are two body fluids that inhabit the extracellular space and what role does each play in the body? For answers, see Appendix G.

48 108 UNIT 1 Organization of the Body Developmental Aspects of Cells Discuss some theories of cell differentiation and aging. Indicate the value of apoptosis to the body. We all begin life as a single cell, the fertilized egg, and all the cells of our body arise from it. Very early in development, cells begin to specialize. Some become liver cells, some nerve cells, and so on. All our cells carry the same genes, so how can one cell become so different from another? This is a fascinating question. Apparently, cells in various regions of the embryo are exposed to different chemical signals that channel them into specific pathways of development. When the embryo consists of just a few cells (these are the embryonic stem cells you keep hearing about), the major signals may be nothing more than slight differences in oxygen and carbon dioxide concentrations between the more superficial and the deeper cells. But as development continues, cells release chemicals that influence development of neighboring cells by triggering processes that switch some genes off. Some genes are active in all cells. For example, genes for rrna and ATP synthesis are on in all cells, but genes for synthesizing the enzymes needed to produce thyroxine are on only in cells that are going to be part of the thyroid gland. Hence, the secret of cell specialization lies in the kinds of proteins made and reflects the activation of different genes in different cell types. Cell specialization leads to structural variation different organelles come to predominate in different cells. For example, muscle cells make large amounts of actin and myosin, and their cytoplasm fills with microfilaments. Liver and phagocytic cells produce more lysosomes. The development of specific and distinctive features in cells is called cell differentiation. During early development, cell death and destruction are normal events. Nature takes few chances. More cells than needed are produced, and excesses are eliminated later in a type of programmed cell death called apoptosis (ap o-to sis; falling away ). This process of controlled cellular suicide also eliminates cells that are stressed, no longer needed, injured, or aged. How does apoptosis work? In response to damaged macromolecules within the cell or to some extracellular signal, mitochondrial membranes become permeable, allowing cytochrome c and other factors to leak into the cytosol. These chemicals, in turn, trigger apoptosis by causing activation of a series of intracellular enzymes called caspases. These enzymes are normally dormant, but when they are activated, they unleash a torrent of protein digestion activity within the cell, destroying the cell s DNA, cytoskeleton, and so on, producing a quick, neat death. The apoptotic cell shrinks without leaking its contents into the surrounding tissue, detaches from other cells, and rounds up. Because the dying cell releases a chemical (lysophosphatidylcholine) that attracts macrophages, and sprouts eat me signals, it is immediately phagocytized. Cancer cells do not undergo apoptosis, but oxygen-starved cells do so excessively (heart-muscle and brain cells during heart attacks and strokes, for example). Apoptosis is particularly common in the developing nervous system. It is also responsible for carving out fingers and toes from their embryonic precursors. Most organs are well formed and functional long before birth, but the body continues to grow and enlarge by forming new cells throughout childhood and adolescence. Once we reach adult size, cell division is important mainly to replace short-lived cells and repair wounds. During young adulthood, cell numbers remain fairly constant. However, local changes in the rate of cell division are common. For example, when a person is anemic, his or her bone marrow undergoes hyperplasia (hi per-pla ze-ah), or accelerated growth (hyper over; plas grow), so that red blood cells are produced at a faster rate. If the anemia is remedied, the excessive marrow activity ceases. Atrophy (at ro-fe), a decrease in size of an organ or body tissue, can result from loss of normal stimulation. Muscles that lose their nerve supply atrophy and waste away, and lack of exercise leads to thinned, brittle bones. Cell aging occurs and it accounts for most problems associated with old age. Cell aging is a complicated process with many causes. The wear-and-tear theory attributes aging to little chemical insults and formation of free radicals, both of which have cumulative effects. For example, environmental toxins such as pesticides, alcohol, and bacterial toxins may damage cell membranes, poison enzyme systems, or cause mistakes in DNA replication. Temporary lack of oxygen, which occurs increasingly with age as our blood vessels clog with fatty materials, leads to accelerated rates of cell death throughout the body. Most free radicals are produced in the mitochondria, because these organelles have the highest metabolic rate. This finding implies that diminished energy production by radical-damaged mitochondria, due perhaps to an increasing burden of mutations in mitochondrial DNA, weakens (and ages) cells. X rays and other types of radiation, and some chemicals, also generate huge numbers of free radicals, which can overwhelm the peroxisomal enzymes. Vitamins C and E act as antioxidants in the body and may help to prevent excessive free radical formation. With age, glucose (blood sugar) becomes party to cross-linking proteins together, a condition that severely disrupts protein function and accelerates the course of arteriosclerosis. Another theory attributes cell aging to progressive disorders in the immune system. According to this theory, cell damage results from (1) autoimmune responses, which means the immune system turns against one s own tissues, and (2) a progressive weakening of the immune response, so that the body is less and less able to get rid of cell-damaging pathogens. Perhaps the most widely accepted theory of cell aging is the genetic theory, which suggests that cessation of mitosis and cell aging are programmed into our genes. One interesting notion here is that a telomere clock determines the number of times a cell can divide. Telomeres (telo end; mer piece) are strings of nucleotides that cap the ends of chromosomes, protecting them from fraying or fusing with other chromosomes, much like plastic caps preserve the ends of shoelaces. In human telomeres, the base sequence TTAGGG is repeated a thousand times or more (like a DNA stutter). Though telomeres carry no genes, they appear to be vital for chromosomal survival, because each time DNA is replicated, 50 to 100 of the end nucleotides are lost

49 and the telomeres get a bit shorter. When telomeres reach a certain minimum length, the stop-division signal is given. The idea that cell longevity depends on telomere integrity was supported by the discovery of telomerase, an enzyme that protects telomeres from degrading. Pegged as the immortality enzyme, telomerase is found in germ line cells (cells that give rise to sperm and ova), but it is barely detectable or absent in other adult cell types. CHECK YOUR UNDERSTANDING 4. What is apoptosis and what is its importance in the body? 5. What is the wear-and-tear theory of aging? For answers, see Appendix G. Chapter Cells: The Living Units 109 In this chapter we have described the structure and function of the generalized cell. One of the wonders of the cell is the disparity between its minute size and its extraordinary activity, which reflects the diversity of its organelles. The evidence for division of labor and functional specialization among organelles is inescapable. Only ribosomes synthesize proteins, while protein packaging is the bailiwick of the Golgi apparatus. Membranes compartmentalize most organelles, allowing them to work without hindering or being hindered by other cell activities, and the plasma membrane regulates molecular traffic across the cell s boundary. Now that you know what cells have in common, you are ready to explore how they differ in the various body tissues, the topic of Chapter 4. RELATED CLINICAL TERMS Anaplasia (an ah-pla ze-ah; an without, not; plas to grow) Abnormalities in cell structure and loss of differentiation; for example, cancer cells typically lose the appearance of the parent cells and come to resemble undifferentiated or embryonic cells. Dysplasia (dis-pla ze-ah; dys abnormal) A change in cell size, shape, or arrangement due to chronic irritation or inflammation (infections, etc.). Hypertrophy (hi-per tro-fe) Growth of an organ or tissue due to an increase in the size of its cells. Hypertrophy is a normal response of skeletal muscle cells when they are challenged to lift excessive weight; differs from hyperplasia, which is an increase in size due to an increase in cell number. Liposomes (lip o-sōmz) Hollow microscopic sacs formed of phospholipids that can be filled with a variety of drugs. Serve as multipurpose vehicles for drugs, genetic material, and cosmetics. Mutation A change in DNA base sequence that may lead to incorporation of incorrect amino acids in particular positions in the resulting protein; the affected protein may remain unimpaired or may function abnormally or not at all, leading to disease. Necrosis (nĕ-kro sis; necros death; osis process) Death of a cell or group of cells due to injury or disease. Acute injury causes the cells to swell and burst, and induces the inflammatory response. (This is uncontrolled cell death, in contrast to apoptosis described in the text.) CHAPTER SUMMARY Media study tools that could provide you additional help in reviewing specific key topics of Chapter are referenced below. = Interactive Physiology Overview of the Cellular Basis of Life (pp. 62 6) 1. All living organisms are composed of cells the basic structural and functional units of life. Cells vary widely in both shape and size. 2. The principle of complementarity states that the biochemical activity of cells reflects the operation of their organelles.. The generalized cell is a concept that typifies all cells. The generalized cell has three major regions the nucleus, cytoplasm, and plasma membrane. The Plasma Membrane: Structure (pp. 6 67) 1. The plasma membrane encloses cell contents, mediates exchanges with the extracellular environment, and plays a role in cellular communication. The Fluid Mosaic Model (pp. 6 66) 2. The fluid mosaic model depicts the plasma membrane as a fluid bilayer of lipids (phospholipids, cholesterol, and glycolipids) within which proteins are inserted.. The lipids have both hydrophilic and hydrophobic regions that organize their aggregation and self-repair. The lipids form the structural part of the plasma membrane. 4. Most proteins are integral transmembrane proteins that extend entirely through the membrane. Some, appended to the integral proteins, are peripheral proteins. 5. Proteins are responsible for most specialized membrane functions: Some are enzymes, some are receptors, and others mediate membrane transport functions. Externally facing glycoproteins contribute to the glycocalyx. Membrane Junctions (p ) 6. Membrane junctions join cells together and may aid or inhibit movement of molecules between or past cells. 7. Tight junctions are impermeable junctions. Desmosomes mechanically couple cells into a functional community. Gap junctions allow joined cells to communicate. The Plasma Membrane: Membrane Transport (pp ) 1. The plasma membrane acts as a selectively permeable barrier. Substances move across the plasma membrane by passive processes, which depend on the kinetic energy of molecules, and by active processes, which depend on the use of cellular energy (ATP).